ZOOLOGY AND BOTANY, MICROSCOPY, ETC. 
539 
systems of prisms are next considered. Then under the head of limi- 
tation of the rays and the properties of optical instruments dependent 
upon it, the questions of perspective, magnification, penetrating power, 
brightness of images, the aperture and limits to the resolving power of 
optical instruments are passed in review. 
These general theories form the foundation for the special theory of 
optical instruments, the most important of which, viz. the eye, projecting 
systems, lens, Microscope and telescope, are then discussed in detail ; 
for each of them the dioptric effect and the factors on which it depends 
are determined, and, in the case of the artificial instruments, a critical 
and historical review is given of the most important types of con- 
struction. 
The concluding chapter of the book is devoted to a description of 
the methods employed in the determination of the constants of optical 
instruments. 
At the end of each chapter is a list of the literature bearing upon 
the subject which has been discussed. 
On the Subjective Magnitude of the Monocular and Binocular 
Images in the Hand-lens.* — Dr. Yves Delage gives an explanation of 
the increased magnification which results when both eyes instead of one 
only are used in examining an object through a hand-lens of large 
diameter. 
This increase in magnification must evidently be a subjective phe- 
nomenon, since the retinal image of the second eye is equal to that 
of the first. 
In such a question of magnification, then, there are two things to be 
distinguished : the real magnitude of the retinal image and the subjective 
sensation of the magnification. 
As regards the first point. According to the text-books the lens 
should be placed so that the image is formed at the minimum distance 
of distinct vision. This may be an advantage, but is not a necessity, for 
between eye and object there is a considerable range of distance in which 
the lens will furnish sharp images ; and this is also the case if the eye 
is displaced with respect to the lens. Similarly for each fixed distance 
of lens from the eye, there is a series of positions of the object giving 
sharp images and different magnifications ; and finally there is a series 
of positions of the head giving distinct images. The real magnification, 
then, depends on the relative positions of three factors : object, lens, and 
eye. 
Now for a given position of the lens L L' (fig. 78) with its foci F F 
and of the object 0 O', the image 1 1' has a position and magnitude fixed 
and independent of the eye which perceives it. 
This image can be considered as a real object seen by the eye with- 
out the lens. It is such that any ray I C passing from any one of its 
points to the eye and meeting the lens in K is real in its part K C which 
represents the refracted ray corresponding to the incident ray 0 K. 
This image 1 1' can be seen by the normal eye from an infinite distance 
up to the punctum proximum. 
As the distance of the eye increases, the angle beneath which it sees 
* Arch. d. Zool. Exp., i. (1893) pp. vi.-xiii. 
2 p 
1893. 
