BY THE PRESIDENT. 
121 
existence by the old experiment of looking at two spots of ink on a 
piece of paper on the same level and about three or four inches 
apart. We hold the paper in front of ns at arm’s length and draw 
it towards us, and as we fix say the right eye on the spot to the 
left, we find the spot to the right disappears and then becomes again 
visible when the paper is closer to us. So large is the blind spot 
that the experiment can be varied by slowly walking backwards 
from two plates about three feet apart fixed on a wall. Mariotte 
discovered this phenomenon and amused Charles the Second by 
showing him how he could see men with their heads cut off, 
for while the image of the head of a man could be made to fall on 
the blind spot, that of the figure fell on the surrounding retina and 
was visible. There are other blind spots, or rather blind streaks 
caused by the distribution of the blood-vessels over the surface of 
the retina. The larger vessels shut off the light altogether, and the 
smaller ones reduce its quantity. If we make a hole with a needle 
in a card, and look through it at a bright sky, moving it a little 
from side to side, the bright spot will disappear as its image falls 
on one of the blood-vessels. We perceive the shadows of these 
blood-vessels, which are called Purkinje’s figures, by facing a 
dark wall and holding a candle close to the outer side of the eye, 
so that the light falls obliquely into it. It is only with some little 
difficulty that we can discover these gaps in the field of vision by 
means of the above experiments, and the reason is not because we 
have two eyes, and that what we do not see with one eye we see 
with the other. Persons blind with one eye do not notice these 
defects, which are rectified by the constant movement of the eye, 
and by the attention being generally directed to the image thrown 
on the yellow spot where the vision is most acute, and where there 
are no such imperfections. 
The question, however, naturally arises, How is it that we are 
not conscious of a blank in the field of vision ? But what character 
could this blank assume ? It could not be that of a black patch, 
because on the blind spot there are no structures to enable us to 
see anything, any more than there are on the soles of our feet. 
We find on carefully noting our experiences in the above experi¬ 
ments that the field of vision closes in, just as if there were no 
blind spot at all. 
Purther, when we buy a microscope or telescope, we expect it to 
be achromatic, in other words that it should give an image with 
a sharp outline free from any fringe of colour. We require an 
instrument which will bring to the same focus the different rays of 
colour seen in the spectrum, and which constitute white light. 
