Professor Draper's Description of the Tithonometer. 407 



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 

 insulated by cobalt blue glass, is marked as unity; the centre 

 of the yellow, insulated by the same, being marked 3 ; the in- 

 tervening region being divided into two equal spaces, and di- 

 visions of the same value carried on to each end of the spec- 

 trum. 



As instruments will no doubt be hereafter invented for 

 measuring the phaenomena of different classes of rays, it may 

 prove convenient to designate the precise ray to which they 

 apply. Perhaps the most simple mode is to affix the name of 

 the ray itself. Under that nomenclature the instrument de- 

 scribed in this paper would take the name of Indigo-tithono- 

 meter. 



There is no difficulty in adapting this instrument to the 

 determination of questions relating to absorption, reflexion 

 and transmission. Thus I found that a piece of colourless 

 French plate-glass transmitted 866 rays out of 1000. 



Description of the Instrument. First, of the glass part.— 

 The tithonometer consists of a glass tube bent into the form 

 of a siphon, in which chlorine and hydrogen can be evolved 

 from muriatic acid, containing chlorine in solution, by the 

 agency of a voltaic current. It is represented by fig. 1, where 

 a b c is a clear and thin tube four-tenths of an inch external 

 diameter, closed at the end a. At d a circular piece of metal, 

 an inch in diameter, which may be called the stage, is fastened 

 on the tube, the distance from d to a being 2*9 inches. At 

 the point .r, which is two inches and a quarter from d, two 

 platina wires, x and y, are fused into the glass, and entering 

 into the interior of the tube, are destined to furnish the sup- 

 ply of chlorine and hydrogen ; from the stage d to the point 

 b, the inner bend of the tube, is 2*6 inches, and from that 



