1887.] 



On Photometry of the Glow Lamp. 



257 



violet of the spectrum, and consequently the vertex of the parabola 

 should lie further towards the blue of the spectrum than it does 

 with the visual rays. The method of testing was as follows, 

 i An ordinary da/rk slide, A, carrying a sensitive plate, was placed in 

 grooves, CC, attached to a board, D, against which the slide (when its 

 front B was drawn out) could be raised or lowered as occasion 

 required by means of a rack and pinion motion working by the 

 handle D. 



Fig. 



E is a projection in which a slot, H, is cut, and through which a 

 card having a square aperture, K, can slide. K can be covered by 

 means of a cardboard screen. The bottom of the plate in A is first 

 brought opposite the aperture K, which is placed opposite the number 

 marked 1 on E. The lamp is placed 4 feet away, the volts and amperes 

 noted, and exposure is given to the small square of the plate seen 

 through the square in K for any time which may be fixed upon. The 

 slide A is lowered, so that a fresh portion of the plate is brought 

 opposite to K (K being covered up) a different current passed through 

 the lamp, and another exposure given, and so on. When the top of 

 the plate has been arrived at by the motion of A, the card F is moved 

 till K is opposite 2, and the same procedure repeated. Six to ten 

 exposures can be made in the same row. 



When the second row is exhausted, K is placed opposite 3, and 



