THE ELEMENTS OF MICROSCOPY.—THE HUMAN EYE. 301 

Now, beside these errors, there are others to which the micros- 
copist should devote special attention ; they are caused by small 
opaque particles existing in the transparent media of the eye-ball. 
These cast their shadow on the retina, and produce images which 
appear to exist outside the eye. These extra-retinal images often 
appear as globules, bacterioid-shaped bodies, or strings of minute 
pearls, and may be studied by directing the eye to a sheet of 
strongly illuminated opal glass, through a small aperture made with 
a fine needle in a piece of thin blackened cardboard. (Fig. 31.) 
When the microscope is used in a vertical position, these glob- 
ules often gravitate to the centre of the cornea, and even after pro- 
longed use of the inclined tube an observer may often be perplexed 
by the layer of mucus, or a lachrymal discharge covering the surface 
of the cornea. 
Just a few words as to 
colour perception. Colour 
is a special sensation excited 
in the retina by rays of a 
definite wave length, and 
the reason why certain ob- 
jects are presented to our 
view with colour is that 
when white light falls upon 
a given surface, some is ab- 
sorbed, the remainder be- 
ing reflected. If the green 
rays are reflected, then the 
object appears green, and 
if the red rays are alone 
reflected, then the object 
will be red. Merge 
The generally accepted theory of colour perception is based on 
the assumption that three kinds of nerve fibres exist in the retina, 
the excitation of which produces sensations of red, green, and violet, 
and that modifications of these three sensations yield all inter- 
mediate tints. 
This theory will explain some of the phenomena of colour 
blindness—if the nerve fibres which should give their special sensa- 
tion are paralysed, or are wanting, the sensation only of the com- 
plementary tint will be transmitted with all the defects of the eye ; it 
must not be forgotten that many phenomena consist more in errors 
of judgment than in absolute error of form or sensation. 
Now in regard to errors of judgment, we must admit that all our 
estimations are made by comparison. In magnitude we are guided 
by the size of the retinal image as determined by the visual angle— 
for position we must have some starting point, and as for distance, 


