250 Capt. W. de W. Abney and Mr. Gr. S. Edwards. 



on the slit of the collimator, producing a bright image when the slit 

 had a width of of an inch. The bottom half of the slit was closed 

 by a shntter when the gas spectrum was photographed. To know 

 the locality of the part of the gas spectrum impressed, a second 

 spectrum of the electric arc was photographed below the gas spectrum, 

 lithium and sodium being volatilised, to give a sufficient number of 

 fiducial lines. The plate was then withdrawn from the slide and 

 placed in an apparatus by which a series of small square portions of 

 the plate lying parallel and below the last-named spectrum could be 

 exposed at will. 



The exposures of this series of squares varied between 3| seconds 

 and 5 minutes, generally being 3|, 7J, 15, 30, 60, 90, 120, 180, 240, 

 and 300 seconds. The exposure was made to the light from an 

 Argand paraffin lamp placed at 6 feet from the plate. When the 

 height of the flame was kept constant, no practical difference in 

 illumination was found, and no variation was found during a series of 

 exposures if the lamp were allowed to attain a constant temperature 

 by burning ten minutes. The plate thus impressed with the various 

 images was developed in the ordinary manner with the proper 

 developer. 



If a gelatine plate were used, ferrous oxalate was usually employed ; 

 whilst acidified sulphate of iron was employed if a collodion wet plate 

 were being experimented with. It has not been thought worth 

 while to record all the measurements of the various plates, but a selec- 

 tion has been made of the most important results. It may be 

 remarked that only 5 per cent, addition of bromo-iodide of silver to 

 a bromide of silver emulsion sufficed to shift the place of maximum 

 sensitiveness of the plate from the blue towards the green, as shown 

 in the diagram. After development the plates were fixed as usual 

 and dried, and were then ready for measuring. The following diagram 

 shows the apparatus employed for the measurement, the description 

 of which is taken from a previous paper by one of us. 



A is the source of light — gas, paraffin, or other lamp ; B is a lens 

 of about 9 inches focus, used as a condenser ; C is a double frame for 

 carrying the negative, N", which has- an upward and side motion, so 

 that any part of the negative may be brought in front of the con- 

 denser ; D is a lens on a stand, used to focus the negative on the 

 screen E, which is black except one small square, as shown, where 

 the image of the part to be measured is thrown ; F is one of a series 

 of diaphragms used with D for the purpose of sharpening the image 

 and reducing its brightness when required ; H is the rod used to cast 

 the shadow on the white patch ; Gr is a flat mirror reflecting a beam 

 also on E ; K is the rotating apparatus placed in the path of the light 

 reflected from to diminish it at pleasure; M is the small electro- 

 motor which drives K. The rod H is so placed that the shadows 



