2 Tue Microscope. 
seen in horizontal and longer in vertical position provided the 
distances in both cases are equal. Although the disturbance of 
clear and distinct vision may not be directly perceptible to the 
healthy eye under ordinary circumstances, it will become quite 
apparent if the pupil is unusually enlarged, as is the case in the 
dark, and the eye is directed to an isolated object the light 
power of which is not sufficient to cause much reduction in the 
size of the pupil by contraction. In cases of a high degree of 
astigmatism the prismatic colors will appear at the edges of ob- 
jects in certain directions, according to the direction of the as- 
tigmatic axis, and add to the indistinctness. 
Having thus given a brief account of the nature of astigma- 
tism and its interfering influence to the perfect and natural use 
of the eye, and the remedy of the defect of the same by the use 
of spectacle glasses of peculiar construction, it remains to con- 
sider the relation of the astigmatic eye to the use of optical in- 
struments. 
The telescope and microscope serve to increase the power 
of the eye, not only by simply showing us an object on an en- 
larged scale and with a wider angle of view, but also at the 
same time—by means of increased quantity of light—enabling 
us to distinguish shades of color or close lines at a smaller angle 
of view than the unaided eye is capable of doing. This latter 
property of the optical instrument we call resolving power. It 
is dependent on the angular aperture of the instrument and is 
directly proportionate to it. It gauges the distinguishing capa- 
city of our eye to an extent which it has no opportunity to de- 
velop under ordinary conditions. In other words, the telescope 
and microscope not only increase the angle of view, that is, 
magnifying the object, but also the angular aperture, and con- 
sequently the resolving or separating power of the eye. To 
those unacquainted with the use of optical instruments the 
amount of magnifying power may seem to be the most impor- 
tant, if not the only, condition of the capacity of the instrument, 
but the experienced manipulator knows too well that high mag- 
nifying power is of no value without a corresponding amount of 
resolving power. A telescope of six inches aperture will sep- 
arate a double star that a two-inch glass is incapable of showing 
any trace of separation in, even though both instruments are of 
