418 Lord Blythswood and Dr. E. W. Marchant. 



Now to determine the absorbing powers by measurement of these 

 plates, a special form of photometer was devised for the purpose, as 

 shown below, fig. 3. 



Fig. 3.— Plan. 



Ground gLass. 



Candler) 



:'?//: 



Base. 



Groove in which 



Black dks.phra.gm with slits 



Fig. 4.— Elevation. 



r—Z . 





(77 

 



, I — I — 1 .... T. ... 1 ,1 .. f .... 1 .... 1 .... I 

 12 34 dl 6789 







• 

 i 



S 







^G 



A— Plate. 





* 



i 







i 



i * 



i 



i 



j 



B 







B 



A A is a piece of wood about 10 inches by 6 inches, fixed to the base 

 BB, and fitted at the back with grooves, so that a plate can slide from 

 one end to the other along the grooves, figs. 3 and 4. G is a vertical 

 slot cut in the wood, into which a diaphragm is fitted, pierced by 

 three narrow slits, as shown in fig. 4, special precautions being taken 

 that the three slits were all of the same width (0*5 mm.); H is a 

 horizontal slot, cut as shown, to the bevel side of which a scale, SS, is 

 attached. CC, fig. 3, is a blackened screen, placed so as to screen the 

 eye from all light, except that which appears at the slits in G. 



A piece of ground glass or white paper was placed behind the slit 

 so as to form a uniformly illuminated background. 



The plate was shifted along its slide until the intensity of the illumi- 

 nation of the slits 1 and 2 (say) was equal, the reading on the scale 

 opposite the edge of the plate was then noted, a series of values was 

 taken and the mean value calculated. The scale was so placed that 

 the scale reading gave the distance from the end of the strip corre- 



