Sir John Conroy on a new Photometer. 



425 



£ actual size. 



position shown in the figure, and pieces 

 of white paper, 3 centim. by 3 centim. 

 (filter-paper was tried ; but ordinary 

 white writing-paper not too highly 

 glazed seemed most suitable), held 

 against the hypotenuse of each of the 

 triangular prisms by india-rubber 

 bands. 



It is of course essential that the light 

 should be incident upon both papers 

 at equal angles, and that the papers 

 should be so placed that no light can 

 be reflected from one to the other. It 

 is desirable that both papers should 

 be cut from the same sheet, and that the surfaces on which 

 the light is incident should originally have formed one side of 

 that sheet. 



A rectangular board, similar to that to which the prisms 

 were fixed, was fastened to the top of the prisms by two 

 screws; and to the edges of this board four strips of card, in 

 three of which square apertures had been cut, were fixed, and 

 the whole arrangement painted both externally and internally 

 a dead black. 



In order to adjust the papers, or replace them by new ones, 

 it is merely necessary to withdraw the two screws in the top 

 board and lift it off, together with the sides of the box. 



The edge of the front paper coinciding with the middle line 

 of the box, the photometer could be used with either side 

 uppermost; and in order to be certain that the illumination of 

 both papers was entirely due to light incident directly upon 

 them, measurements of the relative intensity of two similar 

 paraffin- lamps were made with the photometer in both posi- 

 tions; and it was found that the readings were identical. 



The photometer was compared with a Bunsen's disk by 

 placing it at the end of a horizontal board furnished with a 

 scale, and along which a paraffin-lamp was arranged to slide. 

 A Bunsen's disk, in an ordinary form of support with two 

 inclined looking-glasses, could be screwed to the end of the 

 board, to which three stops were so fixed that, when the disk 

 was removed and the new photometer placed against the stops, 

 the middle line of the box was in the same vertical plane as 

 the disk had been. 



A paraffin-lamp was placed on either side of the photo- 

 meter, the position of one remaining constant, whilst that of 

 the other was altered uniil the illumination was equal, and the 

 distance of the latter read off, in centimetres, on the scale. 



