406 Professor Draper's Description of the Tithonometer. 



be measured out. The principle of the modification is easily 

 apprehended. If half the surface of the lens be screened by 

 an opake body, as a piece of blackened cardboard, of course 

 only half the quantity of rays will pass which would have 

 passed had the screen not been interposed. If one-fourth of 

 the lens be left uncovered, only one-fourth of the quantity 

 will pass; but in all these instances the focal image remains 

 the same as before. By adjusting, therefore, upon the wooden 

 frame of the lens, two screens the edges of which pass through 

 its centre, and are capable of rotation upon that centre, we 

 shall cut off all light when the screens are applied edge to 

 edge, we shall have 90° when they are rotated so as to be at 

 right angles, and 180° when they are superposed with their 

 edges parallel. Thus by setting them in different angular 

 positions, we can gain all quantities from 0° up to 180°, and 

 by removing 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 ■propor- 

 tional to the quantity of incident rays. 



I have stated in the commencement of this paper, that the 

 action upon the tithonometer is limited to a ray which corre- 

 sponds 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 : — 



