ACHROMATIC COMPOUND MICROSCOPES. 
191 
contrary ; but one in which both aberrations are destroyed should give the 
outline and the lines distinct together. Even some good glasses, however, have 
a defect, against which we should be on our guard ; that in certain directions 
of the light they are liable to show lines on an object which do not really exist. 
It is observable, that provided the aperture of the glass remains open, the 
central pencil of light admitted behind many transparent objects may be 
limited to a very small one without greatly impairing their sharpness ; parallel 
lines or spots spread closely over a flat surface often remaining plainly visible 
in this case, which at a far less amount of contraction by a stop behind the 
object-glass cannot by any management be made to appear. The reason of 
this seems to be, that both reflection and refraction of a part of the rays take 
place at such objects, by which the pencil is spread out on leaving them to a 
much increased angle in its progress to the glass. 
The relation between the aperture of microscopic object-glasses, even of the 
same focal length, and the pencil of light admitted by them, will vary much, 
according to differences in their thickness, their combination &c. ; and as 
aperture is valuable only in proportion to the pencil it admits, the latter would 
seem to be the circumstance the more deserving attention of the two. It is so 
often erroneously estimated, that I will mention a simple mode of ascertaining 
it, which will be found pretty accurate. 
Fix a piece of paper on a table, and on it place the microscope with its body 
horizontal, and one of the eye-pieces on ; set a candle on a level with it a few 
yards distant ; then having directed the body of the instrument so far on one 
side of the candle, as that the light from it shall bisect the field vertically, 
leaving half of it dark, trace on the paper a line corresponding to the side of 
one of the legs. Now, taking the focus of the object-glass as a pivot, turn the 
microscope horizontally to the other side of the candle till the opposite half of 
the field only is illuminated, and mark again on the paper the position of the 
side of the leg. The measure of the^angle traversed shown by the two lines is 
that of the pencil of light. 
In the remarks which follow, the term correction is used to imply the effect 
produced by the denser concave lens of a compound object-glass upon the 
aberration of its convex. Thus as in a simple convex lens, the rays which 
pass through it near the circumference have their foci shorter than the more 
