ZOOLOGY AND BOTANY, MICROSCOPY, ETC. 
541 
/, and this will be the case unless the lens is very thick. It follows 
from this that the myope can see with the lens more details than the 
emmetrope, since he is able to make p' less. 
On the other hand, however, the magnification defined by g is less 
for him than for the emmetrope, for - = - 
O i 
which shows that — is a maximum when p' is. 
_ p' O' + /) p' + f 
p'f f ’ 
This is only an apparent 
contradiction, for the service rendered to the emmetrope by the instru- 
ment can be greater than that rendered to the myope, without the latter 
ceasing to keep the advantage over the former. 
Now to consider the second point, viz. the subjective sensation of the 
magnification of images. Besides the magnitude of the retinal image 
we have as another element the distance. All objects seen under the 
same visual angle ought to appear equal, but we feel them more or less 
great because we refer them more or less far in the angle. Thus a doll 
of 15 cm. seen at 1 metre appears smaller than a woman of 1 m. 50 seen 
at 10 metres, although they furnish equal retinal images. So inversely 
a man appears as large at 5 metres as at 10. To what distance then are 
the images furnished by the lens referred ? 
In monocular vision the images furnish no direct indication as to 
their situation on the visual ray. Indirectly we are guided by com- 
parison with other objects in the field of view and also by the effect of 
accommodation, but this element of judgment is not very precise. In the 
case of the lens, if it alone influenced us, we should refer the image to 
its true distance, but other effects intervene which cause us to modify 
our impression. According to the author’s experience we refer the 
image nearly to the position occupied by the object which furnishes it. 
Thus in examining with a lens an object on a table, the field of the lens 
does not appear to be sunk into the table as would be the case if the 
retinal image was referred to the distance of the virtual image. The 
feeling of the continuity of the parts seen in the lens with their pro- 
longation beyond the field dominates the less intense impression of the 
effect of accommodation. The resultant sensation is doubtless a com- 
promise between the organic sensation and the corrected sensation, but 
much nearer the latter than the former, at least with persons who 
frequently make use of the lens. 
Thus the image seen with the single eye is always estimated below 
its true dimensions. When, however, both eyes are opened, the con- 
vergence of the optic axes furnishes instantly a precise and intense indica- 
tion which dominates all the preceding vague approximations ; the image 
is referred to its true distance and consequently appears greater. Thus 
in fig. 79 the rays starting from the point N which, without the lens, 
would make an angle C x N C 2 , form after refraction the smaller angle 
Cj M C 2 . The eyes have the same direction as if 0 O' were at 1 1'. 
With a single eye C the image is referred in the angle a to the distance 
C P, and I x P measures its apparent magnitude ; while with two eyes 
C x and C 2 it is seen beneath the same angle at a distance C M, and its 
apparent magnitude is represented by I M. 
2 p 2 
