MICROSCOPIC OBJECTS. 115 



them as opaque objects, in order to exhibit the hnes, 

 the scale must be brought a httle within the focus, and 

 the illumination carefully arranged. As transparent 

 objects, they are much more easily managed. They pre- 

 sent a mottled sort of colour, composed of the brightest 

 caraiine, mixed with purple, blue and yellow, and their 

 lines are distinctly seen. As the lines on some of these 

 scales are of easy resolution, it wUl not be advisable to 

 trust every specimen as a test. The small ones fi'om the 

 legs of the Brazihan Beetle are the most difficult, and 

 many of these require the most rigid adjustment of the 

 focus and illumination to resolve the hnes ; and the 

 shghtest tremor, though not enough to occasion any 

 sensible dancing, (as a carriage at a distance,) is sufficient 

 to render them invisible. 



" The lines and markings on certain vegetable tissues, 

 and many others too numerous to name, may also be em- 

 ployed as proof objects. The reason for making a selec- 

 tion of those above described, has been, in order to render 

 the task of judging of the merits of an instrument by -dif- 

 ferent individuals more simple and satisfactory." The 

 scales from the wing of the PapUio Janira are esteemed 

 by Professor Amici as difficult test- objects. The same 

 distinguished microscopic observer considers the lines on 

 the scEiles from the margin of the under wing of the male 

 Lasciocampa Quercus very difficult of resolution. When 

 the penetration of the microscope is sufficient, the lines 

 appear triple. This appearance is probably due to their 

 tubular structure. 



M 



