COLOUR STUDIES WITH THE MICROSCOPE. 
127 
colour that coincides with similar rules. No one likes, or 
ought to like, heavy masses of colour that are injudiciously 
contrasted with light tints. We often see the ugly result of 
inattention to this rule in wall-papers and carpets, in which 
huge lumps of flowers or fruits, in strong colours, are scattered 
over pale thin grounds; and we also see it when panels and 
borders are so coloured that heavy tones are put where the 
light would look best, and vice versa , in defiance of the rule 
that supports of any kind should look strong enough and firm 
enough for the things supported. But the reader may exclaim, 
“ What has this to do with the microscope ? ” Wait a bit, and 
it will be seen how readily experiments which would occupy 
weeks if made with brushes and paint can be tried in a few hours 
by means of the microscope and its appliances. Before, how- 
ever, proceeding to the experiments, a further, though brief, 
consideration of the subject theoretically will be desirable. 
Colours affect the eye in proportion to their masses and their 
luminous intensity. From physiological reasons the eye is 
affected by the quantity of space a given light occupies in 
estimating its intensity. This is easily proved by looking 
at a watch-face when the light is too faint for the figures to be 
distinguished. If only a small quantity more light would 
make them plain, it can be practically obtained by the use of a 
low-power lens. The lens takes in a larger quantity of light 
than would have entered the pupil without it, and though 
through its magnifying power it spreads the light over a pro- 
portionally larger surface, the effect upon the eye is to make 
every part of that surface look brighter. This is sometimes 
denied, but it is easily proved ; and when using telescopes of 
even no more than three inches aperture, it is often convenient to 
place a delicate neutral tint glass over the eye-piece to moderate 
the blaze of the full moon seen through the instrument. 
Within certain limits, increasing the size of a surface that 
emits a bright colour adds to its apparent brightness, and 
makes a greater strain upon the eye. 
The length of a wave of red light is about 3 9 1 0 0 th of an 
inch ; that of a wave of violet light about y j 0 0 th of an inch, 
and the intermediate colours of the spectrum range between 
these two extremes. 66 The sensation of red is produced by im- 
parting to the optic nerve four hundred and seventy-four mil- 
lions of millions of impulses per second, while the sensation of 
violet is produced by imparting to the nerve six hundred and 
ninety-nine millions of millions of impulse in a second.” * 
When the eye is strongly affected by any one colour, its nerves 
not only vibrate in unison with it, but they are disposed to 
Tyndall, “Notes on Light. 1 
