MICROSCOPIC 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, 
Fj g 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 naked eye, 
or even with ordinary telescopes, as single 
stars; but when instruments of superior 
power are directed to them they are 
resolved, 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. Nebulae 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 greater 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 object, it will assume such an appearance as is shown 
50 
