84 Mr. A. P. Trotter on a New Photometer 



photometer, in which two screens at 45° to the lights* and to 

 the eye are used, but side by side, instead of one behind 

 the other. 



I showed an extemporized photometer of this kind to 

 Mr. Crompton and to Mr. Swinburne on March 26, 1893. 

 Mr. Crompton suggested the use of zinc for the screens, and 

 this gave me the hint to try perforated zinc, to abandon a 

 definite vertical fine of uniform tone, and to select a line or 

 band of uniformity on a screen provided with a number of 

 perforations. I mentioned the idea to Mr. Swinburne on 

 March 27, but had no opportunity of trying it for more than 

 a week. In the meantime Mr. Crompton had thought of a 

 similar expedient, and had used with considerable success a 

 pair of screens, the front one of which was perforated with 

 horizontal slots, and he observed the shading of the slots, say, 

 from bright to dark, and of the bars from dark to bright. 

 Before I had heard of this modification I had tried vertical 

 bars or slots and perforated screens. 



The photometer is shown in fig. 2. It consists of a box 

 mounted on a sliding-carriage on a photometer-bar. A 

 slotted screen and a plain screen are fixed inside, and are 

 observed through an opening from the direction 0. The 

 lights fall on the screens through two openings from the 

 directions L and L x . The lights are arranged exactly 



Fig;. 2. 



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opposite the middle of each side opening, and not in the 

 plane of the photometer-bar as is usual. The lower edge of 



* See Note at end of the Paper. 



