TEST OBJECTS. 467 



the merits of any achromatic combination may be divided into 

 three kinds ; viz., hairs of animals, scales from the wings and 

 bodies of insects, and the siliceous coatings of recent and fossil 

 infusoria, those of the latter kind being the most difficult of all 

 to define. The following list contains some of those that Mr. 

 Topping and others are in the habit of furnishing to their cus- 

 tomers as test objects, they being covered with the thinnest 

 glass, in order that object-glasses of the highest power may 

 be employed upon them : — 



From this list the author has selected a certain number of 

 each class, and highly magnified representations of them are 

 given in plates vi., vii., viii., and ix., which may be looked upon 

 as the finest engraved specimens of these minute structures that 

 have yet been executed, and reflect the greatest credit both 

 on the artist and the engraver, the original drawings having 

 been accurately traced with Mr. Leonard's well-known skill 

 by means of the Camera Lucida applied to the microscope, 

 the power employed with some of them being as high as 2,000 

 diameters. 



Bafs Hair. — This beautiful structure, represented by A, 

 figs. 1, 2, and 3, plate vi., is obtained from a species of bat in- 

 habiting some parts of India ; it is remarkable as presenting a 

 series of scale-like projections, arranged in the form of a whorl 

 30* 



