110 THE Microscope. 
ture; and assuming objectives perfect in the first three quali- 
ties, we will only consider the latter. We say that objective is 
optically most perfect which has the greatest angular aperture. 
Why? Because it will show the greatest number of lines to 
the inch, by which I do not mean only ruled lines, or the much 
abused lines on diatoms, but structure and detail in the ordi- 
nary objects. It is absolutely-necessary to use an objective of 
a given angular aperture in order to see a given number of 
lines, whether it be of one power or another. For instance, if 
you wish to see a line of 1-50,090 inch in width, it is necessary 
to use an objective with an angle of 65°, whether it be a 4, 4, t, 
ort. An objective of smaller angle will not show them, no 
matter how perfect it may be. If the eye-piece be of too low 
a power, the lines may not be distinguishable with the 4 or 4, 
but by using the proper eye-piece they will become so. This is 
not due to any particular quality of the eye-piece, except in so 
far that it gives sufficient amplification to separate the lines 
that the eye may perceive them. It may be the 4 objective 
with a 2-inch eye-piecg, or the + objective with a 1-inch eye- 
piece. This choice depends entirely upon the will of the 
worker, but has its limits. It is restricted on one side. by angu- 
lar aperture, on the other by the power of the objective which 
is used. There is a limit to the increase of amplification by 
eye-pieces, and when we cease to geta distinct image by adding 
higher power eye-pieces, we say it breaks down, i. e., there are 
not sufficient rays from the objective to bear the loss which each 
higher power of eye-pieces inflicts. By taking the same power 
objectives, but larger angle, we find that this can be done, this is 
due to the fact that a much greater quantity of rays are trans- 
mitted, and those which come to the eye are in excess of those 
which were lost by passing through the eye-piece. The other 
difficulty is that as the objectives increase in power, the diffi- 
culty of making the lenses and mounting them increases in the 
same proportion, and faults occur, which at the present day, it 
is impossible to avoid, and which detract considerable from the 
useful qualities. ’ 
Another factor enters here, which is, that the present ordi- 
nary methods of illumination under extremely high powers are 
insufficient to bring out details. I am happy to say that the 
