Fig. 8. 



MICKOSCOl'lC DRAWING AND ENGRAVING. 



If, therefore, the observer were only supplied with a micro- 

 scope, such as would have shown the object as exhibited at D, 

 he would evidently have formed a very 

 incorrect notion of its structure ; and it is 

 accordingly found, that every improve- 

 ment which has taken place has disclosed 

 to us a new order of natural facts. 



In order, therefore, to put the observer 

 in a position to ascertain how far he can 

 rely upon the indications of his instrument, 

 it is necessary to supply him with some 

 objects of known structure, whose details 

 the instrument ought to make visible if 

 it have the power which it claims. 



15. Such objects, which have proved to 

 be eminently useful in microscopic re- 

 searches, and highly conducive to the 

 progress of science, are called TEST- 

 OBJECTS. 



16. In the case of the telescope applied 

 to astronomical researches, similar tests of 

 efficiency are found in countless numbers in 

 the heavens. Double, triple, and multiple 

 stars are the most obvious examples of 

 these. Such objects, as is well known, 

 appear when viewed with the Luked eye, 

 or even with ordinary telescopes, as single 

 stars; but when instruments of superior 

 power are directed to them they are 

 HESOLVED, as it is called, and seen as what 

 in fact they are, two or more minute stellar 

 points in such close proximity, that the 

 space between them is too small to affect 

 the eye in a sensible manner, unless when 

 magnified by artificial means. 



17. Xebula3 supply another order of 

 telescopic test-objects. These appear, even 

 when viewed with telescopes of consider- 

 able power, as small patches of whitish, 

 cloudy light, of gi eater or less magnitude, 

 a character from which they have received 

 their name. 



Such an object is represented, for example, in fig. 9. 

 When, however, a telescope of higher power is directed upon 

 the same oi jeet, it \\ill assume such an appearance as is shown 



