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been able to check the results of an observer so provided. But the case is now so widely altered, by the diffusion of instruments of not dissimilar aperture, that the following notice may be worth publication :
The Earl of Rosse's Great Spiral Nebula in Canes Venatici (or M 51) is a much finer object with the reflector than I had expected from my recollection of its appearance in the 51-inch achromatic. In the latter I could only detect some feeble traces of irregular distribution in the "halo” which the Earl saw as a spiral volute; but with my Newtonian, though the spiral character could not be recognized, the general outline as drawn by him could readily be made out; nor could there be any doubt that what I saw resembled his delineation much more than the half-split ring of Sir J. Herschel. The filling-in with nebulous light between the centre and the ring was very evident, as well as the external projection & p, from which the spiral begins to be wound in, or what we might metaphorically term the mouth of the shell. On this side a small star was very distinct in the nebulosity; it is shown in Nichol's figure (“System of the World,” Plate VI.) but not placed sufficiently south : other minute twinkling points were at first suspected, but could not be verified afterwards. A part of the great convolution equidistant from the two nuclei, and a little f the line joining them, was brighter than the rest of it. A power of 110, already overdoing the light of the achromatic, was here well borne, and perhaps 212 might have been employed, but for the smallness of the field of that eye-piece. Secchi, however, has observed that the object will not bear magnifying.
The place of this surprising nebula is R. A. XIII h. 24m. D.N. 47° 52'. But as many of our readers may not be provided with circles, it may be better to show how it may be found by sweeping. Nearly 20 of a great circle p n Urso Majoris (the bright star at the end of the tail), we notice a 5 mag. star (24 Can. Ven.): 8 p this are three much smaller stars lying in an open triangle, and just p the star at the S. angle (or 20 s, a little p, from 24 Can.) lies the object of our search. It will appear but a feeble patch in an ordinary 11-inch finder; that of my reflector, having a little diagonal mirror, shows it with difficulty ; but having reached the guiding star, if we turn to the lowest power in the telescope, we shall readily get it into full view. The night, of course, ought to be very clear, and without a moon.
A marvellous contrast to this mysterious haze is the great Globular Cluster in Hercules, M 13, to which one always returns
with fresh pleasure and admiration after gazing on feebler objects. The resolution with the reflector far surpasses that in the achromatic, and might be thought almost complete with 450, but for the mention by H. of stars down to the 20th mag. I could not make out the “ dark lanes" figured by Lord Rosse, but there were evidently some more vacant spaces, especially one small roundish opening. The outliers are chiefly of the larger stars, and I could not help suspecting that in this case, as in M 3, referred to in THE STUDENT, No. vi., p. 460, a central globe of smaller stars may be viewed through an external shell of larger and much more thinlyscattered ones. The supposition there made, that the greater stars may be sprinkled through, as well as around, the smaller mass, is perhaps unnecessary, but the point requires closer attention than I have yet given it, and probably superior optical means; and should the idea of an external shell of different character be substantiated, it is possible that it may be found of more frequent occurrence and of some special signification. The caution of Schröter well deserves to be borne in mind, on this and many other similar occasions“With the largest telescopes we still always view the great works of God in a remote background, and we can only hold to sure observations, and to that which follows immediately from them.”
THE EYESIGHT AND THE MICROSCOPE,
BY HENRY J. SLACK, F.G.S., SEC. R.M.S.
I HAVE lately heard of several cases of persons purchasing microscopes, and soon becoming afraid to use them, lest they should permanently injure their sight. Now,
Now, if the instruments they used were of even moderate merit, the fault of not seeing objects comfortably lay entirely with themselves. It often happens that a beginner with a microscope operates chiefly with transparent objects, and floods the field with excess of light. Any of the paraffin lamps in ordinary use for microscopical purposes, or such excellent oil lamps as those which Mr. Pillischer supplies, give an immense deal more light than is wanted to exhibit any ordinary objects properly, either by transmitted or reflected illumination, and when this light is concentrated by a bull's-eye, and reflected by the stage mirror in full blaze, it is by no means wonderful that the eye is speedily fatigued. A few objects may be advantageously shown under brilliant illumination, for the display of remarkable beauty in the variety of colours
they present. The wing case of the diamond beetle and iridiscent minerals belong to this class, and they should be viewed as we look at flashing fireworks, or the lustre of jewels, for a brief space only, and not in a prolonged stare. As soon as it is desired to make out details of their structure, the light should be reduced to a moderate pitch.
There are microscopic difficulties which involve prolonged effort to decipher obscure markings, or indications, with which beginners should have nothing to do, and which experienced microscopists must cautiously deal with if they value their own natural optical apparatus. Men who will sit up night after night, poring for hours over vexatious diatoms, have no right to complain of the microscope if they experience a deterioration of vision. Had they spent the same time in attempting to read very small print in a strong blaze, they would have been equally successful in wearying their visual organs. Such practices are an abuse of the eye, to which, no doubt, a penalty is attached.
The perfection of microscope work consists in its imitation of natural vision. The instrument should extend the range of action of the eye upon small objects; but should not—except for brief purposes of display-materially alter its character. Now, the first thing to be attended to is to keep both eyes open, whether the microscope be used for single or binocular vision. It is unnatural for two-eyed people to shut one eye, and then make a prolonged observation. There are occasions on which it is very desirable to shut one eye for a few moments, as in taking an accurate aim with a rifle, but with the microscope, or telescope, all that is necessary is to acquire the habit of paying attention to the impressions made upon the eye which looks through the instrument, and to disregard what the other may see. Some people have no difficulty in so doing, while others can only succeed if assisted by a little contrivance which many observers have long employed. I mean a shade covered with black cotton velvet, of which several forms have been devised. The simplest, which I have used for many years, is made of a piece of thin cardboard about as big as a small quarto page, covered with black cotton velvet, and pierced with a hole through which the tube of the microscope, just below the eye-piece, is introduced. I have found that everyone upon whom I have experimented, and who felt it difficult to keep both eyes open, and only look with one, could easily accomplish it by this means. There is no doubt that the eyes suffer considerably from the common practice of closing one, while looking through a microscope, or telescope, for any length of time,
with the other, and it is, therefore, well worth while to acquire the more prudent habit I have described.
The next point to be considered is the method of modifying the light, and diffusing it agreeably through the field. artificial light is employed to show transparent objects, it is rarely advisable to throw it as it comes from the lamp, or the bull's-eye, direct upon the object. For low powers and large objects, the best contrivance I know is one which Mr. Browning made by my direction a few years ago. It consists of two discs of glass, ground on one side only. The two ground sides are placed in contact, and the edges cemented, to keep them in position and exclude dust. A freshly ground surface of good glass is remarkably pleasant to the eye; the cool dead white appearance it gives to transmitted light is very agreeable, but its performance is deteriorated by handling the ground surface, or by impact of dust. To keep the surface in a fresh state I adopted the method just described, which works excellently with 4-inch, 3-inch, 2-inch, and 14-inch powers. For two-thirds and half-inch powers, and smaller objects, I take an ordinary slide, and place in the middle of it, on one side, a piece of white foreign post paper, as wide as the slide, and about an inch long, saturated with spermaceti, and covered with a piece of thin glass, to keep it clean. A few thin chips of spermaceti are placed upon the paper, and melted into it over a lamp. When this spermaceti-paper slide is employed, the side bearing the
is turned downwards, and the slide carrying the object placed on the uppermost side. By this means the texture of the paper is kept out of focus.
Those who have an achromatic condenser should use the small stops to moderate the light, as well as to obtain sharper vision. I have recently been in the habit of placing below the condenser a modification of Mr. Rainy's light moderator, which Mr. Browning made to my pattern with some excellent grey glass in his possession. This apparatus consists in a short tube fitting into the sub-stage of my microscope, and capable of going under the condenser, or of being used without it. The bottom of this tube is covered by a brass plate, with a round hole in the middle. Two brass arms, moving upon a pivot, carry two disks of the grey glass, and one or both can be turned so as to cover the round hole, and moderate all the light thrown up by the stage mirror. This contrivance is very effective in reducing glare.
Mr. Collins makes a good moderating diaphragm, by which the aperture through which the light comes can be enlarged, or reduced
to a mere point, at will. Messrs. Beck have adapted to the microscope a very elegant iris diaphragm, imitating the action of the pupil of the human eye, and preserving a nearly round aperture, whether dilated or contracted. These instruments have a double use. They enable the quantity of light to be nicely graduated, and thus save the eye from fatigue, and they permit us to observe with great delicacy the varying degrees of transparency different objects, or different parts of the same object, possess.
With diatom markings, and many other objects, a plan just introduced by the Rev. J. B. Reade, F.R.S., President of the Royal Microscopical Society, will be found very valuable as an aid to distinct and pleasant vision. He places below the object an equilateral prism, with one side parallel to the object. The light is thrown on to another side, by means of a bull's-eye, in parallel rays, and being reflected by the third side of the prism, enters the object with moderate obliquity. It is not easy to explain why this illumination is so remarkably effective as it is found to be, but no one who has seen it in use can avoid being at once delighted and surprised.
There are many objects which can only be properly shown by sending a very strong light through them, and if the whole field is illuminated in the same proportion, the effect is wearisome and painful. In such cases I strongly recommend the eye-piece made at my suggestion by Mr. Ross, and known as my Diaphragm Eyepiece. In this eye-piece four shutters can be adjusted by small milled heads, conveniently situated, so as to leave the entire field open, or to shut off any portion of it that can be bounded by square, oblong, or rhombic figures. By this means the object is, as it were, framed in black, with just as much of light margin as may be desirable. The adjustments are easily made to suit long, thin, round, or square objects, as the case may require.
All the contrivances mentioned are adapted to binocular, as well as to monocular vision.
Various modes of obtaining dark ground illumination, by the spot-lens, parabola, etc., are much to be commended for certain objects, not only for the beauty of the display, but for the comfort of the eye; and a small spot-lens may be advantageously used with the binocular as a light-distributor, with powers that can take in its whole pencil of rays.
It is much more common for persons to injure their sight by the misuse of transmitted light with transparent objects, than for them to experience inconvenience from any excess of reflected illumina