340 Mr. Pritchard's Account of Test Objects for Microscopes, 



especially the French ones, are very easy, and unworthy the 

 title of proofs ; and, as they might be substituted for those I 

 am describing, and thus a common instrument might pass for 

 one of superior excellence, I feel justified in giving this cau- 

 tion*. 



The size of these scales varies from one nine hundredth to 

 one hundred and sixtieth of an inch in length, and, as they 

 decrease in size, become more transparent. They are of dif- 

 ferent forms, but possess a general character, easily recognised, 

 by the want of any sharp angles. Under a microscope not 

 having sufficient penetration, the tissue appears devoid of 

 structure or markings; but, when placed in a superior one, 

 and the illumination properly made, they show a series of 

 lines or cords on their surface, and present a much greater 

 variety in their arrangement than the scales of any other 

 species of insect. Some have the lines straight, as shown in 

 the magnified scales, Plate III. fig. 5. and 6, and have two 

 sets of oblique lines on them, similar to fig. 8f; others are 

 waved and curved, as shown in fig. 7, 9, and 10, while on 

 some of the small ones, as fig. 11, nothing satisfactory has 

 yet been developed. In these figures I have endeavoured to 

 give the appearances which the objects present under the mi- 

 croscope; and it will be observed, on a careful inspection of 

 them, that the lines on fig. 9, 10, and 13, (which are only 

 portions of scales,) are very different from those on fig. 5. 

 and 6, the former ones not being so sharp and defined as the 

 latter. 



As a general rule, it will be found that the smaller the 

 scales the more difficult the test; those in fig. 6, however, 

 cannot be included as tests, as they are very easily resolved. 

 I must not omit to notice, also, that the cords on these scales 

 are loosely attached to the tissue, and are often rubbed off in 

 mounting. Of course it will be fruitless to examine such spe- 

 cimens. Those on which the greatest reliance may be placed 

 are similar to fig. 5, though the same scale will assume all the 

 appearances of fig. 8, 9, 10, and 13. 



Before leaving the subject of the lined objects, I should no- 

 tice, that all objects of similar structure are more or less tests, 

 as the lines on the scales of some beetles, one of which, from 



* It should be remembered that the exhibition of the lines on these 

 scales is only proof of the penetration of a microscope; and unless the out- 

 line of the scale is sharp, the instrument is defective in definition. 



f As in the scales of the Pontia Brassica, only one system of oblique lines 

 can be seen at once; the other system is similar to those in fig. 8, but 

 running in a direction at right angles to them. 



