MICROSCOPY. 



511 



marked on the setting, and in his adjustable 

 ones the collar-graduation indicates in hun- 

 dredths of a millimetre the corresponding thick- 

 ness of cover. 



The comparatively short working-focus of a 

 high-anglea objective is inseparable from the 

 combination selected, but the accompanying 

 want of penetration may be partly relieved by 

 temporarily cutting down its aperture by in- 

 serting diaphragms back of the lenses. An 

 iris diaphragm in a Society screw adapter is 

 screwed in above the objective for this pur- 

 pose. It was proposed by Dr. Koyston Pigott 

 in 1869, and again by Mr. W. H. Bulloch in 

 1878, and finally introduced under the name 

 of " aperture-shutter " by Mr. G-. E. Davis in 

 1882 (" North. Mic.," 1882, p. 13). 



The Ocular. It is one of the curiosities of the 

 microscope that the u negative " ocular of Huy- 

 gens, which consists of non-achromatic lenses, 

 and was early borrowed from the telescope for 

 the microscope (on a mistaken theory of its 

 advantages), has thus far defied both com- 

 petition and improvement, being conceded to 

 be still the most available ocular for general 

 work. Its construction presents no difficulties, 

 if its simple formula be carried out with rea- 

 sonable care; and there is, therefore, no need 

 of sensible inequality in its performance, ex- 

 cept that smallness of the tube chosen may 

 narrow its field of view. In practice, how- 

 ever, marked inequalities are found, due either 

 to careless workmanship or to a disregard of 

 the well - known principles of construction. 

 Some makers even use the same supply of 

 field - lenses for oculars of various powers. 

 Huygenian oculars are commonly made with 

 powers of from two inches (often marked A) 

 to f or -J inch (D or E), the one inch being the 

 power said to be employed by prominent mak- 

 ers in correcting and adjusting objectives. 



Of more or less achromatic oculars, first in- 

 troduced by Kellner, various modifications are 

 now supplied by nearly all makers under the 

 names of orthoscopic, aplanatic, or periscopic. 

 These are mostly positive, the achromatic eye- 

 lens having its lower focal plane in or beneath 

 the thick, double- con vex field -lens. The field- 

 lens also is sometimes achromatic ; and by Mr. 

 Tolles it was made, for micrometric use, in two 

 horizontal sections, balsam-cemented together, 

 with a micrometer-scale ruled, and filled in, on 

 one of the adjacent surfaces where it was out 

 of 'danger of dust or wear. All these give a 

 broad, white field of view, flat and well defined 

 to the edge, and are, therefore, well calculated 

 for the display of large sections or of polarizing 

 effects ; but none of them are fully equal, in 

 defining power and general working character, 

 to the Huygenian form. They are common- 

 ly made of powers of from .! inch to f inch. 

 For high powers, from to or even inch, 

 the solid eye-pieces introduced by Tolles in 

 1855, and soon after adopted by Hartnack, of 

 Paris, are now made by nearly all opticians, 

 and are in general use. These are now made 



FIG. 19. 

 ACHROMATIC 



SOLID 

 OCULAR. 



achromatic by Steinheil, of Munich, a double- 

 convex lens of crown-glass c being cemented in 

 between two meniscus lenses of flint//, as 

 shown in Fig. 19, and grooves being cut around 

 the circumference, and blackened to act as dia- 

 phragms, as in the Coddington. The positive 

 Ramsden ocular, formerly much 

 used for micrometry, has been 

 nearly superseded for that pur- 

 pose by the orthoscopic and the 

 solid. 



In the combining of oculars 

 with objectives, it is still unde- 

 cided whether it is preferable to 

 secure a sufficient variety of 

 powers by means of a large num- 

 ber of objectives, or by the high 

 and low eye-piecing of a few. Dr. Carpenter 

 prefers the two-inch, which is certainly the 

 least fatiguing to the writer and to many oth- 

 ers, and would use even the one-inch only for 

 exceptional purposes, believing that the habit- 

 ual use of strong oculars is more trying to the 

 eyes than that of high objectives. Prof. Abbe 

 selects f inch ( x 15) as the highest really use- 

 ful ocular. On the other hand, some excellent 

 workers prefer oculars from one inch (the low- 

 est furnished with many Continental and some 

 American stands) upward, using objectives of 

 high aperture to bear such amplification. "With 

 the five objectives specified in the last column 

 of the above table, and a full set of oculars, all 

 powers may be 1 easily obtained, from a very 

 low to a very high limit, and lines or points of 

 any degree of fineness known to be visible can 

 be well seen. Some advocates of the forcing 

 policy limit the list of desirable objectives to 

 three; and it has even been claimed .that with 

 a good 3%ths " we can see about everything 

 that can be shown by any objective." 



In respect to size and nomenclature of ocu- 

 lars, important progress has been made through 

 the efforts of the American Society of Micros- 

 copists. At the Detroit meeting in August, 

 1880, a committee upon this subject was ap- 

 pointed, consisting of the president and ex- 

 presidents of the society, to which the succeed- 

 ing presidents have been successively added. 

 This committee was instructed to report a 

 proposal for a standard size of oculars, in or- 

 der that these might be interchanged with fa- 

 cility like objectives ; and also to recommend 

 a system of nomenclature by which oculars, 

 of whatever make, might be correspondingly 

 named and rationally compared. The commit- 

 tee reported, at the Chicago meeting in 1883, 

 a series of resolutions which were allowed to 

 lie over for discussion until the following year, 

 when they were adopted without material al- 

 teration at the Rochester meeting. In this ac- 

 tion the society adopts and recommends 1'25 

 inch as the standard diameter for oculars, but 

 believing that larger and smaller sizes than any 

 one average, however well chosen, will be re- 

 quired and will continue to be made, 1 '00 and 

 1*35 inch were recommended as secondary 



