502 



MICROSCOPY. 



which may he fairly considered as represent- 

 ative of American judgment and ingenuity. 

 These stands are mostly made of medium size, 

 the peculiarities of their mechanism enabling 

 them thus to combine nearly all the simplicity 



FIG. 6. HAND-REST, OPEN. 



LJL 



^_ ^ 



ra 



FIG. 7. HAND-REST, CLOSED. 



and portability of the Continental stands, with 

 nearly all the efficiency and scope of adaptations 

 of the more ambitious English instruments. 

 The " Histological" stand of Mr. William H. 

 Bulloch, of Chicago (see Fig. 8), may serve 

 as an example of this type of stand in a very 

 simple and inexpensive form, yet efficient for a 

 great variety of scientific work. On the other 

 hand, the " Universal " stand of the Bausch & 

 Lomb Optical Co., may be specified as a type of 

 somewhat larger and more elaborate stands, 

 able to use all accessories, and serviceable for 

 all general purposes (excluding specialties hav- 

 ing extraordinary requirements) of microscop- 

 ical research. (The general arrangement of 

 this stand is shown in the cut of one, Fig. 30, 

 bearing an electric illuminator in place of the 

 mirror; while its details of construction are 

 nearly like the corresponding portions in the 

 "arc" stand, Fig. 12.) Somewhat equivalent 

 to these stands are numerous instruments by 

 nearly all American and many English makers. 

 A critical study of the points which such stands 

 have in common will reveal the principal im- 

 provements in stands during the last half-dozen 

 years. 



The body is broad enough to hold oculars 

 giving a liberal width of field with the lower 

 powers; 1'25 inch being the standard diameter 

 of the American Society of Microscopists, 

 and l - 35 the largest standard of the Royal Mi- 

 croscopical Society. Many of these stands are 

 made at or near one of these sizes ; and it is to 

 be greatly desired and reasonably hoped that 

 complete correspondence will be shortly ac- 

 complished, so that a single adapter will make 

 the oculars of all interchangeable. The nose- 

 piece at the bottom screws out, leaving the 

 "Butterfield" broad-gauge screw (one and one 

 fourth inch), for those usei for which the " So- 

 ciety " screw is unavailable. The body is also 

 short, about five or six inches, for comfort when 

 using it in a vertical position on the laboratory- 

 table ; this peculiarity fitting it also to adopt 



the Abbe binocular ocular*. A draw-tube 

 (sometimes there are two) furnishes the varying 

 length of body required in micrometry, or the 

 standard length for ordinary use.* The gradu- 

 ations of the draw-tube should be numbered 

 not from zero, but from 

 the minimum length of 

 the body, so that the 

 reading at any point of 

 draw-tube extension will 

 give directly the total 

 length of body in use at 

 the time. The draw- 

 tube is furnished at its 

 lower end with a "So- 

 ciety " screw adapter, 

 which often serves a 

 good purpose as a dia- 

 phragm, and can be em- 

 ployed on occasion to 

 carry an objective as 

 erector, an amplifier, a 

 polariscope analyzer, a set of spectroscope 

 prisms, or an objective of too low power to 

 be focused in the ordinary way. 



The limb connecting the body with the pillar 

 is of the Jackson model, supporting the body 

 steadily by means of the rack attached to its 

 posterior surface. Ten years ago this model 

 was already adopted by all the prominent 

 American makers, except one who made only 

 stands of the Continental type, though the 

 Ross style, with a transverse bar supporting 

 the body only at its lower end, was still ex- 

 tensively, if not most commonly, preferred in 

 England. This style, though confessedly in- 

 ferior in other respects to the Jackson, was 

 long retained from the great superiority of its 

 fine adjustment to that then used with the rival 

 model ; but the faulty adjustment having been 

 superseded, the Ross bar has at last been aban- 

 doned even by the distinguished firm which 

 gave it a name, and whose fine workmanship 

 and large influence greatly prolonged its use ; 

 and it may, therefore, now be considered obso- 

 lete except for special purposes. 



The coarse adjustment is by a rack whose 



* Ten inches is still called the standard length of body, 

 though rather vaguely or approximately, since no one has 

 undertaken to say where the measurements should begin and 

 end. Perhaps they are most commonly made from the top 

 of the draw- tube to the bottom of the objective. As the focal 

 plane from which the "optical tube-length," as it is well 

 teimed by Prof. Abbe, should be measured, is always above 

 the front lens of the objective, and often above the back lens 

 (" J. R. M. 8.," 1883, p. 816 ; "Proc. A. 8. M.," 1884, p. 183), and 

 as large stands do not ordinarily give ten inches from that 

 plane to the corresponding plane of the ocular, it is evident 

 that the powers resulting from various combinations of ob- 

 jectives and oculars must be, as they practically are, less than 

 their theoretical value, computed on the assumption of ten- 

 inch tube, unless either objectives or oculars or both be spe- 

 cially named, so as to make good the practical deficiency of 

 distance between them. Prof. Abbe's cut, however, seems 

 to do more than represent the practical difficulties of the 

 case; as the positive ocular figured, but rarely used in the 

 microscope, has its focal plane wholly below it and far from 

 any convenient location of the top of the draw-tube ; while the 

 negative ocular, far more commonly used, has its plane at the 

 level of the diaphragm between its lenses, and can be easily 

 and conveniently arranged, as it already is by some makers, 

 to slip into the tube just to that plane (see " Nomenclature of 

 Oculars," infra, p. 512). 





