We will consider these several points and recommendations seriatim, commencing with the stand.
Messrs. Watson & Sons’ Van Heurck model stand so well answers the several conditions laid down by an experienced teacher of bacteriology, that I have no hesitation in presenting it to my readers as a typical instrument, one in every way worthy of the high praise it has already received from those who have worked with it, and whose judgment may be relied upon in every way. The microscope is fully described among Messrs. Watson’s instruments, page 108.
The Stand.—A good firm stand is undoubtedly of the first importance for all high-class work. The steadiness of the instrument and its entire freedom from vibration depends largely upon the form of the stand. I am glad to find Dr. Crookshank in accord with me as to the Ross-Jackson model, one which, in my opinion, has not been entirely superseded by models of a more recent date. Indeed, the latest improvement effected in the Ross-Jackson form, in which attention has been given to the spreading-out of the feet, has converted it into as solid and firm a stand as Powell’s; it is equally free from vibration when placed in the horizontal position.
There are, however, four different forms of stands—the tripod; the plate with double columns; the single column ending in a plate or a bent claw; and the horse shoe. The tripod stand, with cork feet, is by far the steadiest form of model. The single upright pillar support should unquestionably be condemned, as it admits of considerable vibration, and is most inconvenient for laboratory work. The heavy horse-shoe form is compact and firm, and the weight of it can hardly be considered an objection.
The Tubular Body is from eight to ten inches in length, to which is added a draw-tube with an engraved millimetre scale. By extending the draw-tube greater magnification is obtained, but since this is at the cost of definition it should hardly ever be employed in the examination of bacteria. A Triple Nose-piece is doubtless a convenience, saving time which is otherwise spent in replacing objectives of different magnifying powers; there is also less risk of injuring them. Focus should be obtained by means of a rack and pinion coarse adjustment, together with the most approved kind of fine adjustment. The sliding tube cannot be recommended, as the motion may be stiff, encouraging the use of force, which in turn may result in the objective being brought violently into contact with the specimen, thus doing injury to the lens or damage to the preparation; or it may get too loose and readily slip out of focus.
The Stage should be flat and rigid, either rectangular or circular, so long as it is sufficiently large to accommodate plate cultivation. A removable mechanical stage is of great advantage for working with high powers, as a motile bacterium can be constantly kept in view, while one hand is engaged in working the fine adjustment; it may also be employed as a finder, if engraved with a longitudinal and vertical scale, and provided with a stop. The mechanical stage must be removable, so that the stage proper may be free from any attachments when required for the examination of cultures.
Diaphragms.—The plan of using a series of separate discs of different sizes should be avoided, as they are easily lost, and bacteriological investigations may have to be made under conditions in which it is difficult to replace them. A better plan is a revolving plate with apertures of different sizes, but the most convenient form is the iris diaphragm.
The Sub-stage Condenser is as necessary in biological work as in the objective—in fact, the condenser and the objective should be considered as forming one piece of optical apparatus; the microscope must be regarded as incomplete without it.
It is by the sub-stage condenser that the rays of light are concentrated at one point, or on one particular bacterium; for the best definition it is essential that there should be mechanical arrangements for accurately centring and focussing the condenser. All this will be explained and enlarged upon under “Practical Optics.”
In the historical review presented to my readers on the evolution of the modern microscope, I have for the most part relied upon my long and close association, extending over a period of upwards of half a century, with microscopy. I need hardly say I could have very much extended my remarks with pleasure and profit had space permitted, and thereby much increased the number of names of manufacturers, who have well-established reputations for the quality of their work, and whose instruments, more or less complete in design, realise the wants of students and of that large class of present-day workers engaged in microscopical pursuits to whom economy of outlay is almost a first consideration. No valid reason, however, can be assigned for splitting up, as some writers do, the several forms of microscopes into some six different classes, which implies inferiority in mechanical details or finish, whereas the difference wholly consists in luxurious appliances to save time, and in accessories for special work or original research. Before bringing these remarks to a close, it is my wish to direct the student’s attention to one or two points of importance in connection with the use of the instrument, viz.: variations in body-lengths of microscopes, especially between those of English and of Continental manufacture. The optical-standard measurement adopted in this country for the body-tube-length is 10 inches; and for its mechanical, 8¾ inches. That of Continental opticians is, optical-tube-length 7·08 inches, or 180 mm.; the mechanical, 6·3 inches = to 168 mm.