Prof. Draper's Description of the Tithonometer. 223 



tions, we can gain all quantities from 0° up to 180°, and by re- 

 moving them entirely away reach 360°. 



It will be understood that the effect of the instrument is to 

 give an image of a visible object, of which the intensity can be 

 made to vary at pleasure in a 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 in the position D, setting its screens at an 

 angle of 90°. Remove the screen E, and determine the effect on 

 the tithonometer for one minute. At the close of the minute, 

 and without loss of time, turn one of the screens so as to give 

 an angle of 180°, and now the effect will be found double what 

 it was before, as in the following table. 



Table II. — Showing that the indications of the tithonometer are pro- 

 portional to the quantity of incident rays. 



I 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. 



Table III. — Showing that the maximum for the tithonometer is in the 

 indigo space of the spectrum. 



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 



