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X. Some Notes on Photometry. 

 By Silvanus P. Thompson, D.Sc, F.B.S* 



I. On the use of Two Overlapping Screens as an Isophotal. 



THE employment of two opaque screens placed at an angle 

 with one another, and with the observer's eye, to receive 

 the light from the two sources, was made the basis of a 

 photometric method by the author, in conjunction with Mr. 

 C. C. Starling, in 1881. Their photometer was design3d, in the 

 first instance, for electric-light measurements. For this special 

 end any method in which opaque screens are used is to be 

 preferred, in one respect, to those methods in which trans- 

 lucent screens are used; for then it is not necessary to employ 

 coloured glasses when making the comparison, as the opaque 

 screens themselves may be of coloured material, and the choice 

 of opaque coloured material is much more varied than is that 

 of tinted glass. 



In the Thompson-Starling photometer the two surfaces for 

 receiving the light met at an angle of about 70°. The pair 

 of screens was constituted by two pieces of card, either white 

 or coloured, or by two surfaces of some brilliantly-tinted 

 fabric mounted on card, each pair being dropped down into 

 position over a wedge-block. The observer, placing his eye 

 opposite the arete of the dihedral pair of screens so as to 

 view each surface at the same angle, bad to adjust the appa- 

 ratus until the apparent illumination at the adjacent parts of 

 the two surfaces was equal. 



When working with this photometer it was found that the 

 precision of judgment of the eye as to equality of the two 

 illuminations was impaired if by bad workmanship any con- 

 siderable width of blunted edge intervened between the two 



t 



surfaces that should have met with precision. There was a 

 similar defect in the original form of the Bouguer photometer, 

 wherein the opaque partition was continued down to the 

 screen and interposed an unilluminated patch equal in breadth 



* Communicated by the Physical Society : read June 9, 1893. 



