458 MANIPULATION. 



page 347, will be found exceedingly useful. It is, perhaps, 

 as well here to state, that the examination of all morbid 

 structures should be made as soon as convenient after their 

 removal from the body, as changes of form in the softer 

 substances speedily take place ; but if some time has elapsed, 

 the part from which the sections are taken should be at some 

 little distance from the surface, in order that they may be as 

 slightly altered as possible by the action of the air. 



CHAPTER XX. 



TEST OBJECTS. 



For this important class of objects, to which, in a great 

 measure, must be ascribed the rapid advancement towards 

 perfection of the achromatic compound microscope, we are 

 indebted to the late Dr. Goring, who, it is said,* was led to 

 adopt them by reading a passage in the works of Leeuwenhoek, 

 relating to the examination of the scales from the wing of the 

 silkworm moth, the lines on which could not be seen by the 

 draftsman with so low a power as that used by the great 

 microscopist himself. At the time of Dr. Goring's first 

 employment of these objects, he ascertained that the structure 

 of certain of them could be readily made out by some micro- 

 scopes, and not by others; and, inferring that there were 

 some peculiar properties in the lines on the feathers and 

 scales of certain insects which rendered them more difficult of 

 definition than others, he was induced to view them through 

 an achromatic microscope, and was led to the discovery 

 that in it there were two distinct powers, viz., defining and 

 penetrating, and that an object-glass might possess the one 

 almost to perfection, and yet be totally devoid of the other, 

 or might be perfect in both. He subsequently made the 

 important discovery, that the penetrating power depended on 

 • Microscopic Cabinet, by Andrew Pritchard. London, 1832, p. 137. 



