786 
SUMMARY OP CURRENT RESEARCHES RELATING TO 
The image of a point with direct light is a disk of about 19 /x in 
diameter, surrounded by a pale yellow ring (on red and violet ground) 
about 8 fx broad. 
For two oblique pencils, every individual image corresponding to 
the length of arc of the zone in the 
Fig. 87. light parts of the “ Austritts-pupille ” 
is a circular arc of the same angular 
aperture. 
In the following table this angle 
(<£) is given for the zones 0*1, 0*3, 
0*5, and so on. 
h 
<t> 
0-1 
166° 
0-3 
152° 
0*5 
134° 
0-7 
113° 
0-9 
88° 
1-1 
56° 
1-3 
18° 
1-5 
35° 
1-7 
39 
1-9 
38° 
2-1 
33° 
2-3 
25 
2*5 
0° 
The image will be in this case 
represented by a number of arcs, 
which, with the transverse aberration 
( t , are inclosed in the angle <f>, corre- 
sponding to the height of emergence. 
In order to simplify this representa- 
tion of the image, those rays are to be 
taken in which the value of o- remains 
stationary, i. e. the maxima of the 
transverse aberration (e. g. o- D = 24 * 8 
for 10-1/4, o- E = 12 for 13*8/x). 
Representing these as circular arcs, 
it is seen from the figure that with 
two oblique pencils the image is repre- 
sented as a disc DEF with oval 
nucleus, which at E is coloured yellow- 
green, at D wine-yellow (yellow on red 
and violet ground), at F violet (blue 
and yellow-green predominating), and 
is surrounded by a large oval pale- 
yellow ring (dd). 
The same result is obtained if the transverse aberration of all rays 
at the distance 0*1, 0*3, 0*5, &c., so far as they take part in the forma- 
tion of the image, is represented by a number of circular arcs, and the 
