108 
ANNIVERSARY ADDRESS 
The course I propose to adopt in my address is to draw your 
attention to the visual organs of the different groups of the Animal 
Kingdom as adapted to their function, and I shall conclude with a 
reference to the human eye. 
It would he well now to briefly consider Light from a physical 
and physiological point of view. In the first place, do not let us 
think of light as we see it, but of the physical cause of the sensa¬ 
tion, which is a movement propagated by successive waves in the 
ether distributed through the universe, in a manner analogous to 
circles caused by a stone thrown into water. These waves of light 
differ in size just as the waves and the ripples on the ocean. They 
all travel at the same rate, so that the number of waves which 
reach any spot in a given time is dependent on their length, and 
colour-sensations are due to the variations in the number falling 
on the eye in a certain time. The longest waves are invisible to 
ns, and are called dark heat waves. The shortest waves are also 
invisible, and are called dark chemical waves, while the waves of 
moderate length produce the sensation we call light. But these 
latter waves are also not all of the same size. The longest of them 
cause the luminous sensation we call red, and as they become 
shorter, so the sensation is what we call yellow, green, or blue, 
while the shortest waves are violet. We see all these colours in a 
rainbow, and in the light after it has passed through a prism. The 
image so produced is named a spectrum. I have just stated that 
the waves which are longer than the red waves (called the ultra- 
red waves) and those which are shorter than the violet waves (called 
the ultra-violet waves) are invisible, but they can be brought 
within the range of human vision by altering their length. Yet 
we can form no opinion of the colour which the unadapted waves 
would produce if onr eye were able to receive their stimulus, for 
the colour of a wave of light, like the note of a wave of sound, is 
dependent on its length. The rectilineal propagation of light, as 
seen in a sunbeam crossing a room, has led to the expression “ rays 
of light,” and thus we speak of (say) violet rays, as denoting the 
propagation of those waves which give to us the sensation of 
violet light. 
We do not know that our fellow-man, and still less the lower 
animals, see colours as we see them. Experiments have shown, 
however, that these latter are also affected by different colours in 
different ways. The daphnias (water-fleas) prefer to rest under a 
green light rather than under violet, blue, yellow, or red. The 
question now arises, Can some animals see rays which are invisible 
to us ? If so, they may perceive colours unknown to us. Let us 
