Prof. Draper’s Descripti the Tithonometer. 223 
tions, we can gain all quantiti n 0° up to 180°, and by re- 
moving them entirely away 60°. 
It will be understood 1 effect of the instrument is to 
give an image of a visible t, of which the intensity can be 
made to vary at pleasure ina known proportion. — 
In order therefore to prove that the indications of the titho- 
nometer are proportional to the quantity of impinging rays, place 
this measuring lens im the position D, setting its screens at an 
angle of 90°. Remove the screen E, and determine the effect on 
the cco er for one minute. At the close of the minute, 
ithout loss of time, turn one of the screens so as to give 
— ele of 180°, and now the effect will be found double what 
it was : before, as in the following table. 
Tasie []—Showing that the indications of the tithonometer are pro- 
portional to the quantity of incident rays. 
Experiment I. Experiment II. 
Quantities. Observed. | Calculated. “Observed. | Calculated. 
2:18 2:22 2°69 275 
180 427 4:45 575 | 650 
270 6:70 6 67 8:25 8°25 
360 8:99 8-90 11:00 | 11:00 
_ [have stated in the commencement of this paper, that the ac- 
‘tion upon the tithonometer is limited to a ray which corresponds 
in refrangibility to the indigo, or rather, that in the indigo space 
- its maximum action is found. The following table serves at once 
to prove this fact, and also to illustrate the chemical force of the 
different regions of the spectrum. ‘ 
Taste IIL. ey that the maximum for the tithonometer is in the 
ndigo space g the <edis 
Space. Ray. Force. Force. __ 
0 | Extreme red, 33 ~ Blue eatuaee 204-00 
&.| Redy ca pecbO Indigo, 240-00 
2 | Orange, EB Violet, 121-00 
3. | Yellow, 275. st Violet, 72:00 
4 | Green, 10-00 | Violet, 48:00 
5 WGreen-blue, | 54:00 Violet, 24-00 
6 lue, 108:00 Extra spectral, 12:00 
7 | Blue, 144-00 
In this table the spaces are equal ; the centre of the red, as in- 
sulated by cobalt. blue glass, is marked as unity ; the centre of 
