ZOOLOGY AND BOTANY, MICROSCOPY, ETC. 



763 



varied with the temperature. He attributes this to changes in the 

 Canada balsam cement, which seems to possess a hitherto unsuspected 

 variability dependent perhaps on age as well as on temperature. Amon»- 

 other experiments he tested a certain camera objective before and after 

 ten hours' heating at G0° C. ; all the zonal errors were altered, one, e.g. 

 rising from — 0*05 to + 0*56. In very important work, he concludes 

 it would be best to use objectives free from cement. 



Everett, J. D. — On Skew Eefraction through a Lens; and on the Hollow Pencil 

 given hy an Annulus of a very obliquely placed Lens. 



Proa. Roy. Soc, LXXI.(1903) pp. 509-522 (2 plates). 



Source der, H. — TJeber die Geschichte der Technik der Mikroskope. 



[Mainly an historical account of the evolution of modern lenses, inter- 

 spersed with interesting anecdotes.] 



Central Zeit, f. Opt. u. Mech., XXII. (1901) Xos. 19, 20,21, 22. 



(3) Illuminating and other Apparatus. 



Tubeufs Drawing Apparatus.* — This apparatus (fig. 168) is in- 

 tended for drawing objects from nature. By means of a prism an object 

 is so reflected into the eye that its vertical projection on the drawing 

 plane appears erect, a very desirable condition in nature-drawing. On 

 the prism plane turned towards the object, smoked glasses of various 



Fig. 1GS. 



thicknesses can be applied for reducing the brightness of the object. 

 On the prism plane towards the eye there is a small revolving disc, with 

 small apertures for regulating the pupil opening. The prism can be 

 set on a stand at various heights and widths. 



Fuess' Hemispherical Gypsum and Metal Reflectors.f — These 

 reflectors, numbered 8 in the maker's^ catalogue, are intended to be 

 placed on the Microscope stage over an opaque object of very small 

 dimensions. The arrangement is shown in fig. 169. The light coming 

 from the mirror reachesthe white spherical interior of the gypsum, and 

 is thus completely reflected in all directions ; the object being thereby 

 completely and uniformly illuminated without shadows. An opening 



* Leitz' Catalogue, No. 40, Nov. 1902, p. 74; Centralb. t*. Bakt., 1899, pp: 765-6. 

 t Fuess, Special-Liste, No. 74, pp. 4 and 5. 



