424 Sir John Conroy on a new Photometer. 



give absolute measures ; the one I propose describing is no 

 exception to this general rule. 



I had intended to use, in some experiments on the amount 

 of light reflected by metallic surfaces, the ordinary Bunsen's 

 disk ; but I found that, owing to the small size of the beam of 

 reflected light, it was impossible to make any satisfactory 

 measurements with the disks in common use, and after trying 

 various photometric arrangements I finally adopted a modifi- 

 cation of Ritchie's photometer. 



The various forms of shadow-photometers work well ; but 

 as the accuracy of the determination depends on the edge of 

 the two shadows coinciding and yet not overlapping, it is 

 necessary to have some arrangement for altering the distance 

 between the screen and the shadow-producer, which adds to 

 the complexity of the apparatus, except indeed when, as in 

 Mr. Harcourt's photometer for gas-work, the variation in the 

 relative intensities of the two lights is caused by the size of 

 one of the flames being altered, and not, as in those arrange- 

 ments heretofore in use, by altering the distance of the flame 

 from the screen whilst the size is kept constant. 



Ritchie's photometer, as is well known, consists of two 

 pieces of white paper fastened to the adjacent sides of a trian- 

 gular block of wood, each being illuminated by one only of 

 the lights which are to be compared. Finding it impossible 

 so to arrange the apparatus that the illuminated surfaces should 

 be actually in contact, the bend in the paper along the edge 

 of the block separating the two illuminated areas, and therefore 

 interfering with the accuracy of the determination, I placed 

 one of the pieces of paper slightly in front of the other, and 

 overlapping it to a small extent, so that, whilst both were 

 visible to the observer, each was illuminated by one only of 

 the sources of light ; when equally illuminated, the edge of the 

 front paper vanished. 



It was originally intended that the light should be incident 

 upon the surfaces of the paper at an angle of 45°; but it was 

 found that when the light regularly reflected by the paper 

 reached the observer (i. e. when the line of sight and the 

 direction of the incident light formed equal angles with the 

 normal to the paper) it was not possible to make satisfactory 

 measurements. 



After various positions had been tried, it was found that the 

 best results were obtained when the light was incident upon 

 the paper at an angle of about 30° and the line of sight 

 formed an angle of 60° with the normal. 



Two triangular blocks of wood, 4 centim. high, were screwed 

 to a rectangular board about 15 centim. by 10 centim., in the 



