100 
MR. LISTER ON THE IMPROVEMENT OF 
increasing the intensity of the illumination, whereas by darkening the object 
beyond a certain point they may be rendered ill defined, and be at length dis- 
solved. 
These peculiarities are most observable on some opaque objects, (the reflec- 
tion from a very small microscopic globule of quicksilver* offers perhaps the 
best example,) but the same effects are produced on the light received from 
transparent ones ; and the consequent blunting and mingling together of their 
minute details when the object-glass admits but a small pencil of light, gives 
rise to various fallacious appearances. One of the most remarkable is the 
spottiness which some surfaces assume, not unfrequently so much resembling 
small globules as to have been mistaken for them ; an optical illusion having 
thus been the basis of some ingenious speculations on organic matter. 
Such appearances have little place with the finer achromatics, the large 
angle of whose pencil and its accurate correction enable us to magnify their 
image greatly, still discovering something new in our object, before we are 
checked by the circles of diffusion of the effective rays ; but these at last, 
whether proceeding from the causes mentioned or from others to be hereafter 
noticed, form in every microscope the boundary to defining power, except 
where faulty materials or workmanship give it an earlier limit. 
It is the marginal rays which contribute especially to render visible close 
and delicate lines, such as those on the scales of lepidopterous insects, and 
some of the most difficult of these are even best seen when the central light is 
intercepted -j~. 
A glass that is far from correct in its figure will sometimes show lines of 
this description sharply, while the outline of the scale is indistinct, and the 
■ To obtain such, I have always placed a globule on a piece of black glass, and scattered it into 
many by a smart stroke, some of which will be extremely minute. 
f The blue down of the Menelaus and the white of the Cabbage Butterfly (Morpho Menelaus and 
Fieris Brassicae) well deserve the place they have acquired as standard tests, especially on account of 
the respective characters of their transverse tracings. Some of the small oval scales from the body of 
the Twenty-plumed Moth (Alucita hcxadactyla) are among the closest and most elegant in their lines 
that I have seen ; others are very easily resolvable, and the down has such diversity of form as often 
to afford within a short space a ready variety of excellent tests. I he same is to be said of the scales 
of Podura plumbea, of which all are difficult, and some seem to defy all powers of definition. 
It may be observed, that with most objects there is such difference between individual specimens 
nl in me kind, that in general the only safe way to determine between two good microscope , is 
to apply the sam< specimen to both under the same circumstances of power and light. 
