ZOOLOGY AND BOTANY, MICROSCOPY, ETC. 667 



applied on a larger sector-carriage, so that greater range of motion 

 may be allowed for the condenser. For use with our most recent 

 homogeneous-immersion objectives, the motion in arc of M. Nachet's 

 condenser is too limited to develope the fullest power of their aper- 

 tures. The only reason we can assign for the neglect with which 

 Mr. Grubb's sector and M. Nachet's traversing substage were received 

 by microscopists in general, is that in those early days so few objec- 

 tives were produced with apertures large enough to be used effectively 

 in conjunction with such apparatus. 



The intention of all the swinging arrangements — both those 

 above referred to and the others described at pp. 1060-80 of vol. iii. 

 • — is to provide oblique illumination up to the highest limit of trans- 

 mitting power of aperture possessed by our objectives ; also to provide 

 " dark-field " illumination beyond this limit, or opaque illumination 

 above the horizon of the object under examination — together with 

 exact means of registering the inclination of the light, &c., so that the 

 effects may be repeated again with certainty. 



It has, however, been objected to the swinging substage that 

 it is an unnecessary incumbrance, as there are various condensers 

 which convert axial into obliquely incident light up to the utmost 

 practical limits of obliquity. Of those recently constructed with 

 special reference to " immersion " illumination, we may note the 

 Abbe-Zeiss oil-immersion condenser, Powell and Lealand's two 

 forms,* and Stephenson's catoptric immersion illuminator, f In all 

 these devices diaphragms are arranged to exclude a portion of the 

 centre of the pencil, and to allow more or less of the peripheral zone 

 to be utilized, the range of action being still further increased by the 

 eccentric and rotating motions of the substage in which they are 

 used. 



There can be no doubt that to a very large extent these condensers 

 give effects of oblique illumination equal to those obtained with the 

 swinging substage ; but it must be remembered that the latter enables 

 us to regulate the precise angle and amount of light that is most 

 effectual with every variety of objects. No disposition of diaphragms 

 has yet been applied to condensers used in the axial substage to 

 enable us to regulate the amount of light without altering the 

 obliquity. In this respect, therefore, we think the swinging substage 

 is an advantageous addition to the Microscope. A further advantage 

 is the facility with which, by its means, the mirror or a low-power 

 condenser may be used above the stage for the illumination of 

 opaque objects. 



Many experiments conducted with a view to the determination of 

 the most efficient means for obtaining oblique illumination have led us 

 to value the plan suggested by Mr. Bulloch, of Chicago, U.S.A., as 

 shown in Fig. 141 on p. 1078, vol. iii. (1880). Placing the micro- 

 scope horizontally, with the lamp attached to the concentric swinging 

 substage, a hemispherical lens in immersion contact with the base of 



* See this Journal, iii. (1880) p. 147, and p. S30, where one is figured. 

 t Ibid., ii. (1879) p. 36. 



