ZOOLOGY AND BOTANY, MICROSCOPY, ETC. 649 



fields of a binocular Microscope, in use with that power, remarkably well ; 

 hence, as may be inferred, there is no difficulty whatever in illuminating 

 both fields with all lower powers. 



" In combination with the front lens it has an aperture of 110°, and will, 

 in conjunction with a suitable stop, give dark-ground illumination, with 

 1/4 in. objectives, up to 100° of aperture, or with any power intermediate 

 between that and the 1 in. 



" The combined lenses, having a comparatively large aperture, will be 

 found useful in all cases when a pencil of light of large angular dimensions 

 is desirable, which is very seldom. 



" Used with a stop having one or more side-openings, it will give uni- 

 lateral or equidistant beams of light of considerable obliquity. 



" Generally speaking, the stops, when used, are to be placed close under 

 the lenses, but in practice it will be found that placing a large stop at some 

 distance from the back lens will occasionally disclose structure when every 

 other method fails. A unilateral beam of light, for resolving P. angulatum 

 on a dark ground, is best got by placing a suitable stop on the back lens 

 before screwing on the front. 



" Last, and not least, of the merits of this condenser, is the low price 

 at which it can be supplied, and adapted to nearly any Microscope. It has 

 one fault; it is non-achromatic. This defect is not noticeable with low 

 and medium powers. In using the combined lenses with high powers, the 

 defect may be minimized considerably by careful focusing. Using one 

 lens only, the defect will scarcely ever be noticed. As a matter of fact, 

 only the most costly condensers are really achromatic. To make this into 

 a so-called achromatic condenser would increase its cost, and render it 

 useless for many purposes." 



Nachet's Camera Lucida for Magnifiers.* — This apparatus, shown 

 in fig. 168, consists of a glass cube A, formed of two prisms, one of which 

 has an hypothenuse surface 



gilded on Prof. Govi's Fig. 168. 



method. This is sufficiently 

 transparent to transmit the 

 rays from the object C to 

 the eye at O at the same 

 time as it reflects also to the 

 eye the rays from the paper 

 B and mirror M. The 

 doublet or single lens is 

 at L. The images are of 

 two different tints, the one 

 seen through the gold film 

 being emerald green and 

 that seen by reflection 

 yellow. The difference of 

 colour is said to be of advantage in making clearly visible the point of the 

 pencil. 



M. Nachet supplies the apparatus in connection with the stand, fig. 169. 



The instrument can also be used to reduce drawings, which are placed 

 under the mirror M and the paper under the lens L ; for this purpose the 

 mirror is made to rotate and an extra low power lens is used. As the 

 smallest movements of the pencil are followed by the lens, these reductions 



♦ Robin's (C.) ' Traite du Microscope,' 2nd ed., 1877, pp. 429-31 (2 figs.). 

 1887. 2 U 



