338 Mr. Pritchard's Account of Test Objects for Microscopes. 



brick-work appearance, are seldom to be seen all over the 

 feather at once. The tissue that covers this scale or feather 

 contains the largest portion of colouring matter, and is often 

 destroyed in removing them from the wing, and along with it 

 the cross striae. In such cases, the longitudinal lines only 

 can be visible. The damaged specimens are easily known by 

 their paleness. 



(3.) (Alucita pentad act ylus, and hexadachjlus.) — The ten 

 and twenty Plumed Moths. — The structure of the wings, or, 

 more properly, plumes of these insects, is so peculiar, that few 

 persons acquainted with entomology are strangers to it. 



The twenty-plumed moth is more delicate in its form than 

 the other. The feathers or scales, employed as proof objects, 

 must be taken from the body of the insect, and not from the 

 plumes or wings. Their breadth is generally greater than 

 their length, and their form is never symmetrical. They are 

 transparent, and about one one-hundred and eightieth of an 

 inch long. The scale is often partially covered by a delicate, 

 uneven, membranous film, which obliterates the lines on those 

 parts. The longitudinal lines are not difficult to resolve, but 

 their proximity is such, that they require a considerable power 

 and careful illumination to separate them distinctly. They are 

 elegant microscopic objects, but rather scarce. * * * 



(5.) The Clothes Moth. — (Tinea vestianella.) — These small 

 brown moths possess very delicate and unique scales, requiring 

 some tact in the management of the illumination, to resolve 

 their lines distinctly. I should observe, that it is the small 

 feathers only, from the under side of the wing, that must be 

 considered as tests ; the others are easy. A magnified view 

 of a small one, about one four-hundredth of an inch long, is 

 given in fig. 3. of Plate III. They are readily made out 

 under the single and doublet magnifiers. This is a favourite 

 object with some, who exhibit it as the standard of excellence. 

 I do not consider it very difficult; though it must be admitted, 

 to bring out the lines sharp and clean, requires an excellent 

 instrument. 



(6.) PontiaBrassica* (Leach.) — The pale slender double- 

 headed feathers, about one eightieth of an inch long, having 

 brush-like appendages at their insertion, obtained from some 

 portions of the wing of this large cabbage butterfly, afford an 

 excellent criterion of the goodness of a microscope. Some 

 connoisseurs prefer them to all others, and form an accurate 

 judgement of an instrument by the manner in which it demon- 

 strates this single object. They are easily detached from the 



* This is the Pieris Brassica of Latreille. 



