

Mr. PritchariTs Account of Test Objects for Microscopes, 339 



wing by the point of a quill, but must be gently handled, for, 

 like many others, they are soon mutilated ; indeed I have sel- 

 dom seen them perfect in the ordinary sliders. Those speci- 

 mens which are easily resolved are readily distinguished, being 

 short, broad, and more opake. There are also found, on the 

 same wing, two or three other sorts, but they are unworthy of 

 notice as proof objects. 



In Plate III. at fig. 4*. is represented a sample of the re- 

 gular proof feather. It is very transparent, and has a yellowish 

 tint; the surface is seldom smooth, as indicated at the part a. 

 In the engiscope these inequalities are not so observable, and 

 therefore, when the lines appear strong, the surface is more 

 uniform than in the microscope. This object requires the 

 light more oblique than any other of the lined kind. On this 

 account I have seldom been able to see the lines satisfactorily 

 with Dr. Wollaston's illumination, unless the magnifier was 

 much out of the axis of the perforation. If we throw the light 

 of a candle (placed a few inches behind the stage) obliquely 

 on them, they can be seen very sharp. I have seen them in 

 this way with a simple jewel lens, of only one fifteenth of an 

 inch focus. 



(7.) The Podura plumbea. — {Lead-coloured Spring-tail.) — 

 The body and limbs of these insects are covered with scales, 

 which, from their extreme delicacy, require great care in re- 

 moving. They are also very soft, and easily wounded. The 

 fluid which exudes from the injury so completely adheres to 

 the scales as to obliterate all their markings. Hence they 

 must oe cautiously handled. Those who are desirous of pre- 

 serving these insects, should keep camphor along with them ; 

 through omitting this, I once had a large collection of them 

 consumed by a species of mite (Acarus), which had insinuated 

 itself into the box. 



I have never been able to see the lines on them with a power 

 much below 250 (that is, one twenty-fifth of an inch locus), 

 and therefore microscopes of a lower power cannot be expected 

 to show them, except of very superior quality ; for it must con- 

 stantly be kept in mind, that that instrument is the best which 

 exhibits an object with the least amplification^ all other things 

 being equal. 



It is affirmed, by a very acute experimenter, of these scales, 

 that " all are difficult, and some seem to defy all power of de- 

 finition." The latter part of this quotation is perfectly accu- 

 rate; but I differ in the former, because many specimens, 



* The reader should examine this and the other figures with a hand 

 magnifier. 



2X2 



