AMOUNT OF LIGHT. 



191 



transmitted heat would be to expand the gaseous mixture, but the tithonic effect is to 

 contract it. 



854. Next I shall proceed to show that the indications of the tithonometer are strictly 

 proportional to the quantity of rajs that have impinged upon it; a double quantity pro- 

 ducing a double effect, a triple quantity a threefold effect, &c. 



855. A slight modification in the arrangement (Jig. 104) enables us to prove this in 

 a satisfactory way. The lens, D, being mounted in a square wooden frame, can easily 

 be converted into an instrument for delivering at its focal point, where the sentient 

 tube is placed, measured quantities of the tithonic rays, and thus becomes an invaluable 

 auxiliary in those researches which require known and predetermined quantities of titbon- 

 icitv to be measured out. The principle of the modification is easily apprehended. 

 If half the surface of the lens be screened by an opaque 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 then: edges parallel. Thus, by setting them in different angular 

 positions, we can gain all quantities from up to 180, and by removing them entirely 

 away, reach 360. 



856. 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 pro- 

 portion. 



857. In order, therefore, to prove that the indications of the tithonometer are pro- 

 portional 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 n. 

 Skewing Out Ike Indication* of At TiOunometer are proportional to the quantity of Incident Ray*. 



858. I have stated in the commencement of this paper, that the action 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 : 



