ZOOLOGY .VND BOTANY, MICROSCOPY, ETC. 



651 



(2) Eye-pieces and Objectives. 



Watson and Sons' Parachromatic Objectives.* — Fig. 89 shows the 

 construction of the low-power objective of this series. The construc- 

 tion of the i-in. and -fa-in.. has been given. f The 

 working distance of these objectives is greater 

 than usual, for example, the distance between the 

 front lens of the 1-in. and |-in. objectives and the 

 object is about the same as their focal length, and 

 the ^-in. has a working distance of more than 1 mm. 

 This working distance permits the £-in. objective 

 to be used in connexion with the hasmocytometer, 

 a great boon for clinical laboratory work, as those 

 who are given to blood counting know. The 

 immersion objective has been specially designed p IG# 89. 



and entirely re-computed for laboratory work, 

 and is particularly adapted for the examination of stained specimens. 



(3 1 Illuminating- and other Apparatus. 



Drawing with the Camera-lucida.J — F. Brocher discusses the 

 causes of eye-fatigue frequently experienced in the use of the camera- 

 lucida. He shows that the cause lies in the difficulty of seeing at the 



Fig. 90. 



same time two objects at different optical distances. Thus if the object 

 to be drawn is at A, at a distance of 1 m., and the paper and pencil are 

 at H, at a distance of 80 cm. (fig. 90), the eye has to exert two different 

 degrees of accommodation. To see A distinctly the surface of the 

 crystalline lens must modify its curvature by one dioptrie, while to see 

 H it must modify it by 3 ■ 33 dioptries. It is impossible that the crystal- 

 line lens can exert these two different radii of curvature at the same 

 time. Hence the sense of eye-fatigue. If A is to be seen distinctly, 



* Catalogue, 1910-11, pp. 86-9 (3 figs.). 



t See this Journal, aute, p. 226. 



j Bull. Soc. Zool. de Geneve, 1908, pp. 105-14 (7 figs.). 



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