ZOOLOGY AND BOTANY, MICROSCOPY^ ETC. 



463 



Fig. 103. 



Nachet's Dark-ground Illuminator.— This apparatus (fig. 103) consists 

 of a truncated cone of glass, the base of 

 which has the outer zone ground oflf to a 

 spherical curve, leaving a central plane disc, 

 which is blackened to exclude light. This 

 cone is mounted in a cylindrical tube, with 

 its base upwards, which is applied in the 

 substage after the manner of the usual 

 Continental cylindrical diaphragms, and 

 racked up close to the transparent object. 

 Parallel rays striking oa the conical sur- 

 face are refracted to the lenticular zone, 

 and thence condensed on the object. M. A. 

 Nachet, by whom the apparatus is con- 

 structed, states that " it should be used only 

 with low powers having an angle of apertura less than that of the illumi- 

 nator." 



Quimby's Lamp-shade.*— Mr. B. F. Quimby's " illuminator " or lamp- 

 shade is intended to be used with the Griffith Club Microscope. It consists 

 of three pasteboard cylinders, accurately fitted one within the other — the 

 external revolving on the middle, the inner being removable. All three 

 cylinders are pierced anteriorly by a round aperture ; the middle piece 

 having also a slot. With the inner cylinder removed, the external piece 

 may be twisted one way or the other, the pencil of light coming through 

 the opening thus regulated ; or, in the examination of diatoms, the slot may 

 be used. The inner surface of the second cyliader is white, but for the 

 convenience of those who prefer a black background, the inside of the third 

 cylinder is of that colour, and this may be slipped into the illuminator 

 whenever a dark surface is required. The middle cylinder is surrounded 

 at its lower margin with a brass collar, to which a short tube is attached. 

 Into this tube fits the lamp rod, while the illuminator rests on the rod 

 controlling the light. 



Van Heurck's Comparator.t — Dr. H. Van Heurck has derived the 

 idea of his comparator from the instrument devised by M. Inostranzeflf 

 for comparing the colours of minerals.! The latter instrument, though 



Fig. 104. 



essentially practical, is insufficient for diatoms, as the field is partially 

 intersected and a black band, where the prisms join, prevents perfect 

 approximation. Moreover, it is preferable that the diatoms should be 

 apposed not in their whole length but with half the length of the valve. 



* The Microscope, vii. (1887) pp. 56-7. 



t Bull. Soc. Belg. Micr., xiii. (1886) pp. 76-8 (2 figs.). 



X See this Journal, 1886, p. 507. 



