ZOOLOGY AND BOTANY, MICROSCOPY, ETC. 1065 



some days or weeks after it has been taken out of the glass tube, and 

 by combining a convergent and a dispersive cylinder it was possible 

 to construct a small Galilean telescope. 



The path of rays through cylinders formed by coaxial shells of 

 varying index is also investigated by Dr. L. Matthiessen, with special 

 reference to the eyes of different insects,* who agrees with Professor 

 Exner that the spherical ends of the facet cylinders have very little 

 to do with their action as lenses, which is to be attributed solely to 

 the variation in optical density. He is, however, of opinion that the 

 cylindrical lenses of the cornea are not composed of coaxial cylinders, 

 but of successive shells, like the chambers of Orthoceratites, the 

 refractive index diminishing from one to another in the direction of 

 their convexity. 



In a subsequent note f Professor Exner says that the cylinders of 

 varying density which were at first made of gelatin have now been 

 constructed of glass at the Jena Glastechnisches Laboratorium. At 

 present their optical density can only be made to diminish from the 

 circumference towards the axis. They act as dispersive lenses, and 

 give clear images when they are free from cavities. The only 

 marked defect is the double refraction which duplicates the image 

 near the borders of the field of view, and indicates that the variation 

 of density is not quite regular along the radii of the cylinder. 



Definition of Hairs, "Test Rings. "J — In articles on "Micro- 

 scopical Advances, Ancient and Modern," Dr. G. W. Eoyston-Pigott 

 says that, he considers the " advances of the accuracy and power of 

 the Microscope is well shown in the well-developed structure of 

 hairs. A favourite object figured in antiquated books is the hair of 

 the Indian bat. Quekett represents it as frilled with a kind of coronet 

 of small hairs, ringed at regular intervals, leaving the intermediate 

 transparent quill exposed." With an "oil-immersion l/12th, and a 

 large angle in the oil condenser, instead of frilled hairs, which are 

 purely imaginary, a beautifully serrated cup, with concave notches, is 

 seen, and edges as black as jet, ornamenting the whole of the stem 

 at equal intervals. After so many years of observation of this object, 

 this result is perfectly startling, and throws a strong doubt uj)on 

 innumerable accepted appearances. The black boundary edges are 

 very nearly 100,000th thick." ' 



As to " test rings " he says that " when a brilliant white disc in 

 diatoms can be detected, it is generally accompanied by a jet black 

 marginal ring all round the spherule ; and in brilliant spherules 

 1/40,000 in. in diameter, this black ring has been frequently estimated 

 at l/6th of the disc, or 1/240,000 in. thick. This ring plays so 

 important a part in the definition of diatoms, cells, and molecules, that 

 I shall ask leave to call it the spherule test ring, or, shortly, the test 

 ring ; for, if a glass giving 800 diameters will not show it in a minute 

 spherule (l/90,000th), it cannot be rated as of the finest quality." 



* Exncr's Repert. d. Physik, xxii. (1886) pp. 333-53 (10 figs.). 



+ Arch. f. d. gesamrnt. Pliysiul. (Pfliiger), xxxix. (1886) pp. 244-5. 



X Eng. Mech., xlii. (18S5) pp. 331-2 (14 figs.). 



