III.-On the Comparative Steadiness of the Ross and the Jackson Microscope-stands. By W. B. CARPENTER, M.D., F.R.S.

(Read before the ROYAL MICROSCOPICAL SOCIETY, March 9, 1870.) In most of the older Microscopes the Body was a fixture, and the focal adjustment was obtained by giving motion to the Stage. This plan, however, was very soon abandoned when the improvement of the Microscope, in its Mechanical as well as its Optical arrangements, was seriously taken in hand by men of real constructive ability; and the Stage being made a fixture, two different modes were adopted for supporting and giving motion to the Body, of one or the other of which nearly all the different patterns devised by our now numerous Makers may be regarded as modifications. The one in which the Body is attached at its base only to a transverse Arm, borne on the summit of a racked Stem, I have elsewhere termed the Ross model; not because Mr. Ross could in any sense be considered its inventor, but merely because he was among the first to employ it, and his original patterns are now in general use with extremely little modification. The other, in which the Body, having the rack attached to it, is supported for a great part of its length on a sold Limb, to the lower part of which the Stage is fixed, may with more propriety be distinguished as the Jackson* model; since it was originally devised by Mr. Jackson, and was thenceforth almost uniformly adopted by the Firm which may be considered as the representative of his ideas.

It has always appeared to me that the Jackson model is so obviously preferable mechanically, that if it had been introduced before the Ross model had come into use, it would have been the one more generally adopted; and having lately had an opportunity of comparing the performance of two instruments, one constructed on the Ross and the other on the Jackson model, under peculiarly trying circumstances, and having found my previous opinion most fully confirmed, I have thought it well to bring my experience in this matter before those whom it most especially concerns, namely, Microscope-Makers and practical Microscopists. In order that the bearing of that experience may be rightly understood, it will be desirable in the first instance to examine the conditions on which tremor of the Microscopic image depends.

When the building in which the Microscopist is at work is thrown into vibration as a whole, as by the passage of a heavilyladen cart in the street outside,-or the floor of the room in which he is seated is made to vibrate by the tread of a person crossing it,the Microscope and the observer move together; and if the frame

In the last edition of my Microscope' I inadvertently designated this as the Lister model, having supposed it to have been devised by Mr. J. J. Lister.

Journal,

of the Microscope were perfectly rigid, there would be no tremor of the image. For this tremor is the result, not of the vibration of the Microscope as a whole, but either (1) of the difference between the vibration of the Body as a whole and that of the object on the Stage; or (2) of the difference between the vibration of the two extremities of the Body, the ocular and the objective.

Now it scarcely seems to me possible to conceive a method of construction which should be more favourable to this differential vibration, especially at the ocular end of the Body, than that which is adopted in the Ross model. The long tubular body, fixed only at its base, is peculiarly subject to it; and although the oblique stays with which it is sometimes furnished diminish the vibration of the tube, they by no means prevent it. The transverse arm and the stem which bears it, each have a vibration of their own; and it is obvious that the nearer to the fixed point of the whole systemwhich, in this arrangement, is the part of the racked Stem embraced by the tube that carries the Stage-the flexure takes place, the greater will be the vibration of the Eye-piece, which is at the greatest distance from that fixed point. The only mode in which this vibration can be kept in check, is the giving great solidity to the Stem, the Arm, and the Body, especially the two former; and this, while objectionable on account of the cumbrousness which it imparts to the Microscope-stand, is by no means effectual for its purpose; as every Microscopist knows to his cost, when using very high powers under any condition but that of the most perfect stillness of the support.

On the other hand, in the Jackson model, the support of the Body along a great part of its length reduces to a minimum the vibration of the tube, and the consequent differential vibration of the eye-piece; and even in those modifications of it in which the tube has but a short bearing, as the support is given to it in the middle of its length, instead of at its lower extremity, the vibration equally affects its ocular and its objective extremities. The form of the Limb makes the Body much less liable to vibration as a whole, than when supported on the transverse Arm and vertical Stem of the Ross model; and as there is no fixed point from which such vibration can commence, increasing in extent with the distance from that point, the Body and Stage are much more likely to move together, such motion imparting no tremor to the image.

In the 'Porcupine' Expedition for the Exploration of the Deep Sea, in which I took part last summer, microscopic inquiry had to be carried on under conditions very different from those which obtain on shore. When our ship was lying-to under sail, even if the swell was sufficient to produce considerable pitching and rolling, the motion, being imparted equally to the Microscope as a whole and to the Observer, did not produce any tremor of the image; and

the only difficulty lay in the maintenance of the observer's own position, which was most effectually secured by firmly grasping the leg of the table (which was fixed to the floor of the cabin) between his knees. When the ship was going under "easy steam," with either a fair wind or a light contrary breeze, there was enough general vibration to produce a considerable differential vibration in any Microscope liable to it, and thus to occasion a decided tremor in the image even when only moderate powers were employed. But when we were steaming with full power against a head-sea, the general vibration became so great as to be the severest test of the mechanical arrangements of our Microscopes. Now, it happened that whilst my own instrument a portable Binocular Microscope weighing less than seven pounds, which is my usual travelling companion-is constructed on the Jackson model, Professor Wyville Thomson was provided with an instrument of about the same scale, but heavier by some pounds, made upon the Ross model; and we thus had an opportunity of fairly testing the two plans of construction under circumstances peculiarly critical. The difference in their performance was even more remarkable than I had anticipated. I found that I could use a 1-4th-inch objective on my own Microscope, with an even greater freedom from tremor in the image than I could use a 2-3rds-inch objective on Professor Wyville Thomson's. In fact the image "danced" very perceptibly in the latter, even when the 14-inch objective was in use.

Now I purposely abstain (for obvious reasons) from naming the Makers of these two instruments. But I think it well to say this much, in order to meet the possible objection, that the difference lay rather in the workmanship of the two instruments than in their plan of construction, that the advantage, if any, lay on the side of the Ross model. And my own very decided conviction is, that the adoption of the principles of the Jackson model would be decidedly advantageous, alike for first-class Microscopes, in which the steadiness of the image when the highest powers are being employed ought to be a primary consideration,-for those second-class instruments, which are intended, at a less cost, to do as much of the work of the first-class as they can be made to perform, portability being here of essential importance, and for those third-class instruments in which everything has to be reduced to its simplest form, so as to permit the greatest reduction in their cost.

IV.—A New Method of using Darker's Films.

By EDWARD RICHARDS.

(Read before the ROYAL MICROSCOPICAL SOCIETY, March 9, 1870.)

I AM desirous of bringing before the notice of the Society a new and more exact method of working with Darker's Films of Selenite.

The instrument consists of three cells of brass cut out on the edge so as to leave eight projections, two of which resemble the end of an arrow denoting the positive axis, two semicircular at right angles denoting 90°, and four smaller which stand for 45°, which as signs cannot be mistaken; and are rotated in three separate arms while under a polarizing microscope; and if the Selenite films are turned round till their positive axes coincide they give the sum of their combined retardation.

If any be turned till their positive axes are at 90°, or the semicircular projections to the positive axis or arrow-head of the others, the lesser number is subtracted from the greater.

For instance when the positive axis of the ths is placed at right angles to the positive axis of the ths, the sum of the difference only is obtainedths.

If the 4th is now added with its positive axis coinciding with the positive axis of the ths, seven quarters are obtained; but if placed to coincide with the positive axis of the ths, five quarters is the result.

Therefore, by subtracting by the semicircular projection or

adding by the arrow-head any number from 4th to 13ths, undulations can be retarded, which include all the colours of the spectrum; and I think the simplicity of this arrangement will be self-evident to all.

I do not know of any method in which the number of quarters in use can be seen so plainly as in this: it is no use to have thirteen quarters in the Selenite films unless we can have them under command, to know what suits our object best. Messrs. Beck's arrangement is well suited for this form of cell (which I think is all that is necessary), and the achromatic condenser can be used without difficulty. But in almost all the new methods of late, toothed wheels have played an important part; and I have added six to mine, the object of which is to rotate any number of quarters simultaneously. They are put in gear by a small milled head at the side.

The instrument, including the cutting of the teeth of the wheels, having been done by me (an amateur), it will be a sufficient apology for the way in which the brass work is finished.