158 
DE. T. E. EOBINSON AND ME. T. GETTBB’S DESCEIPTION OP 
circle divided by a vertical diameter into two semicircles, one of which is illuminated 
by plain, the other by polarized light. Supposing them equally bright, if a piece of 
glass be interposed in the plain beam, its semicircle becomes fainter, and the brightness 
of the other must be reduced so as to equal it by turning the eyepiece, in which is a 
Nicol’s prism. The ratio of the intensities is known from the position ( 6 ) into which it 
is turned. Or the equalization may be effected by changing the distance of one of the 
lights ; and the intensities are as the inverse squares of the distances. I used this 
occasionally as a check, but do not consider it as accurate as the other. The circle of 
the eyepiece has two verniers reading to five minutes : this is sufficient ; for an error of 
2 ,- 5 in the angle will in its maximum effect produce one in the measure of intensity of 
only 7 in the fourth place of decimals, while the uncertainty of the eye in estimating 
the equality of the illuminations is thirty-five times as great. I shall return to this im- 
mediately. 
The photometer is fixed on a board furnished with parallel guides in which the base 
of the moveable light slides. A block for supporting glasses is also fitted to these 
guides, and its upper surface is ruled with lines parallel to them, so that any object can 
be surely placed on it in a given position. The lights were petroleum lamps with the 
edge of the flame turned towards the photometer : when the wick is not too high, and 
they have been burning for about twenty minutes, they give an intense and tolerably 
constant light. The verniers are adjusted to read 0 when the polarized semicircle dis- 
appears entirely, and are set to 90° when the lights are adjusted to equality. This 
equality is verified for each observation ; then for any azimuth ( 0 ) the intensity of the 
light transmitted by the glass = sin 2 6 . If the lights be of the same colour, the bounding 
diameter disappears completely and the process is comparatively easy ; but if there be 
any notable difference of tint, it is much less satisfactory. In such cases I found, as 
Frauxhofer had done long since, that one must make this diameter as faint as possible 
— a fact which implies that the two beams, whether equal in intensity or not, make 
nearly the same impression on the eye. 
I must add that within the last few months my eyes have ceased to be fit for such 
delicate work, and that most of these comparisons have been made under my inspection 
by my Assistant, Mr. Charles Faris, and occasionally by Mr. Howard Grubb, both 
practised and good observers. 
In examining object-glasses, they were placed in contact with the emeried disk of the 
photometer, so that no correction is required for any convergence of the light due to 
them. The results for them are given in the following Table I. 
Description. 
Aperture. 
Eocus. 
Intensity. 
Obs. 
in. 
in. 
a. Triple object-glass 
2-75 
48 
0-5497 
15 
b. Double. 
3-80 
63 
0-5962 
5 
c. Double 
3-25 
48 
0-6567 
d. Double 
6-50 
96 
0-6772 
12 
e. Double 
5-50 
0-7928 
24 
f. Double, inner surface cemented... 
5-0 
0-8739 
24 
g. Double, cemented 
12-0 
222 
0-8408 
24 
