MICROSCOPIC DRAWING AND ENGRAVING. 



insects ; the striae and dots upon which could be seen with more- 

 or less distinctness, according to the excellence of the instrument. 

 It was to these that the name test- 

 lg ' objects was first applied. 



22. As the microscope has been 

 improved in its power from year to- 

 year, these test-objects have in- 

 creased in number ; new details of 

 structure being developed by every 

 increase of power and efficiency in 

 the instrument. A certain list of 

 such objects has been agreed upon 

 by general consent, and prepared for 

 sale by the makers, consisting of 

 hairs, scales, and feathers of insects ; 

 as, however, it is not my present 

 purpose to enter into any 1 explana- 

 tion on the subject of microscopic 

 tests, except so far as may be neces- 

 sary to elucidate one of the uses of 

 microscopic engraving, it will be 

 sufficient here to give a few examples of these test-objects. 



23. There is a little insect, vulgarly called the silver-fish, or the 

 silver-lady, of which the proper entomological name is the Lepisma- 

 Saccharina ; it is usually found in damp and mouldy cupboards, 

 and in old wood- work, such as window-frames. The silvery lustre 

 from which it takes its vulgar name, proceeds from a coating of 

 scale-armour, with which its entire body is invested. These scales, 

 when detached from the insect, and examined with a microscope, 

 present a beautiful striated appearance ; their magnitude varies; 

 one, whose length is the 1 14th, and width the 170th part of an inch, 

 is shown in fig. 19, as it appears in a good microscope, magnified 

 400 times in its linear, and therefore 160000 times in its super- 

 ficial dimensions. 



The scale, as here shown, is divided along the middle of its 

 breadth, by a sort of geometrical axis, on either side of which the 

 structure is perfectly similar. A regular series of striated lines 

 diverge from this axis, at an angle of about 45, intersected by 

 another series, very nearly parallel to the axis. 



The divergent striae are very slightly curved; the concavity 

 being presented downwards, and the longitudinal ones ought to 

 appear with a microscope to stand out in bold relief, like the 

 ribs seen on certain shells ; they are more closely arranged as they 

 approach the lower part of the scale, and become more prominent 

 as they are more separated in proceeding upwards. 

 62 



