122 
ANNIVERSARY ADDRESS 
This is effected in optical instruments by using lenses of various 
kinds of glass, blow, on looking at a spectrum at some distance, 
we find we cannot focus at the same time the red and violet rays. 
Again, if we make a narrow slit in a card and place it in a glass 
coloured violet with cobalt oxide, which shuts off the yellow and 
green rays, and then look at a point of light through the covered 
aperture, we shall see either a blue centre with a red fringe, or a 
red centre with a blue fringe, according as the image falls in or out 
of the true focus. Yet with ordinary light we do not notice these 
chromatic defects, which, like the shadows of the blood-vessels in 
the retina, are observable under special illumination. 
Another defect in our eye is that the various media through 
which light passes are not of uniform transparency. The lens itself 
is composed of fibres arranged around six diverging axes, and the 
rays we see around stars and street lamps are due to its construction. 
Yet these appearances only arise when viewing illuminations which 
either from their size or distance appear small. The same cause 
leads some people to see two or three new moons instead of one. 
Further, the vitreous humour contains floating bodies, called musca 
volitantes, which are seen as irregular rows of beads, and are 
occasionally inconvenient in microscopic examination. They are 
“the will-of-the-wisps ” of the eye, for they float away as soon as 
an attempt is made to fix the vision upon them. 
The nicety of structure requisite for the eye to perfectly dis¬ 
charge its function is even traceable in the abnormal results 
occurring in such eyes as are of a somewhat defective formation. 
If, for instance, the curvatures of the cornea or of the lens are not 
symmetrical, a difficulty arises in seeing horizontal or vertical lines. 
Some people cannot see steps and yet can see the banisters; others 
can see the banisters and cannot see the steps. But how rarely do 
we meet with such a defective condition of the organ of sight ? 
Again, the human cornea is supposed to be the most perfect pro¬ 
jection-lens in existence, owing to its ellipsoidal surface. To 
produce this form is the thing striven for by opticians,* but hitherto 
it has baffled all their resources. The eye also has an advantage 
over all other optical instruments in its large field of vision. 
Colour Sensations. 
I have already stated that the region of acute vision is limited 
to the yellow spot. The yellow pigment, however, absorbs the 
greenish-blue rays, so that yellowish light falling on that portion 
* See “The Construction of Photographic Lenses,” by C. Beck, ‘Journal 
Soc. of Arts,’ vol. xxxvii, p. 180. 
