ZOOLOGY AND BOTANY, MICROSCOPY, ETC. 
117 
by light of all colours, e. g. by ordinary lamp-light, since the lens has a 
special focus for each special colour. If the slit is exactly in the focus 
of the red light, only the red rays through the two holes will be parallel, 
while the rays of other colours will converge. 
The consequence of this is that the two holes in the screen will be 
seen as two stars shining in spectral colours. The eye in moving in the 
direction abed will first 
Fig. 11. 
see a 
red star and then successively 
an orange, yellow, green, blue, 
indigo and violet one. From b 
to c there will be a short ex- 
tinction of the light; then the 
second hole will come into view 
and will appear at first violet, 
and then the other colours in in- 
verse order. By simultaneous 
observation with both eyes, in 
the direction r r, two equally 
bright red stars will be seen. 
The focus belonging to the red 
rays lies farthest from the lens. 
If, therefore, the slit be brought 
a little nearer the lens, the red 
colour of the two stars will 
change to orange. 
Since for this observation 
with both eyes the distance of 
the holes in the screen must be 
equal to the distance between 
the pupils, two observers with 
different pupil-distances will, 
under otherwise the same con- 
ditions, see different colours. 
If the slit is moved farther 
from the lens than the focus of 
the red rays, then all the rays 
converge, but the point of in- 
tersection of the red rays lies 
farthest from that of the violet 
nearest to the lens. This point 
of intersection is easily found 
by observing with one eye and 
finding the point where both 
stars appear equally bright and 
equally coloured. 
The spherical aberration may also be demonstrated by the same 
method. For this purpose, beside the first pair of holes, a second pair 
may be bored nearer to the centre. The marginal rays in all colours 
will then be found to converge nearer to the lens than those nearer the 
centre. 
