508 



MICROSCOPY. 



well-known expediency of equalizing the use 

 of the eyes, by employing them alternately. 

 It is located farther from the eye than are 

 other shades, for the purpose of admitting con- 

 siderable stray light, and thus diminishing the 

 fatigue caused by continual efforts at adjust- 

 ment of the eyes when one is darkly shaded 

 and the other brightly illuminated; to ac- 

 complish which object more fully, some per- 

 sons cover the black shade with white paper. 

 For the same purpose, Mr. L. Wray, Jr., has 

 proposed to substitute for black shades a trans- 

 lucent screen of white paper (" J. R. M. S.," 

 1884, p. 956). 



The Olucctives. The Wenham " single-front " 

 objective (Fig. 17), the front being often more 



PIG. 17. SINGLE-FRONT OBJECTIVE. 



than a hemisphere, first displaced the Lister 

 style, which had done such excellent service 

 and had become really traditional, and is said by 

 Mr. George E. Davis to be probably the basis 

 of most of the high powers now made (" Pract. 

 Micros.," 1882, p. 52). It is very sensitive to 

 cover adjustment, and is therefore well adapted 

 to use either as dry or immersion, by screw- 

 collar adjustment only. Its smallest lens being 

 single, is also a point in favor of perfection as 

 well as of ease and economy in manufacture. 

 If all makers would as frankly publish their 

 improvements as Mr. Wenham has frequently 

 done, much better progress would be made. 



The "duplex" objective, anticipated as an 

 experiment by Amici and by Mr. Wenham, but 

 first realized as an objective by Mr. Tolles, is 

 adopted for many of the high-power " resolv- 

 ing " lenses of high grade. The duplex front 

 as shown in Fig. 18 * consists of two simple 

 lenses close together, whose aberrations are 

 corrected by the over-corrections of two com- 

 pound lenses behind. Such a system gives a less 

 equable chromatic correction, over the whole 

 width of field, than one whose single front 

 lens is immediately followed by a compound 

 lens ; but it likewise presents greater facilities 

 for increase of aperture. The front lens being 

 approximately a hemisphere, with the object 

 at its center of curvature, the first refraction 

 takes place at the first spherical surface, where 



* This section shows, with the addition of an illuminating 

 prism, p, the famous one-sixth m&de by Tolles, for Mr. Frank 

 Crisp, Hon. Secretary of the Royal Microscopical Society one 

 of the earliest systems in which was demonstrated the possi- 

 bility, as shown by the marginal ray, c d, crossing the optical 

 axis, a 6, at an obliquity (semi-aperture in glass) of 55, of 

 practically exceeding in immersion objectives the theoretical 

 limit (about 82 interior angle) of dry lenses. 



the spherical aberration is- slight and of a 

 nature easily corrected above; and the first 

 refraction subject to considerable spherical 

 aberration takes place at the lower surface of 

 the second lens, where it is limited in amount, 

 and easy of correction, by reason of occurring 

 in the pencil of rays after it has been con- 

 tracted to some 70 to 90, while in dry ob- 

 jectives the enormous aberration caused on en- 

 tering the front lens, where the cone of light is 

 spread out to its widest divergence, is more 

 serious than the sum of all the interior aberra- 

 tions, and is, in the case of wide apertures, 

 theoretically incapable of complete correction. 

 The added hemispherical front, therefore, re- 

 duces the task of correcting a system of ex- 

 treme aperture to the far easier and more 

 practicable task of correcting one of moderate 

 aperture. The front lens of the Tolles system, 

 shown in the cut, is of exceptionally small 

 size, admitting very precise corrections and 

 giving great capacity to bear high oculars, but 

 possessing a very short working focus, and a 

 smallness of lens itself which, in high powers, 

 renders its construction extremely difficult. 

 In the Zeiss lenses, planned by Prof. Abbe, the 

 front lens is larger, giving longer working 

 focus and greater availability for high-power 

 objectives ; an advantage which is gained at 

 some expense in respect of bearing high ocu- 

 lars, and by employing front lenses whose 

 utilized portion is so nearly a hemisphere that 



FIG. 18. DUPLEX - FRONT OBJECTIVE, WITH TOLLES'S 

 ILLUMINATING PRISM. 



the making and setting of them is a difficult 

 mechanical procedure, notwithstanding their 

 larger size. 



Though objectives of very high power are 

 occasionally made, even y^ inch focus b 

 ing announced, the larger and more conserv- 

 ative portion of microscopists evidently i 

 cline, as they always have done, toward a much 

 more moderate limit. It is probably quite safe 



