Mr. A. P. Trotter on a New Photometer. 83 



a horizontal screen of white cardboard, or of paper mounted on 

 glass, having a clear star-shaped hole in the middle. Below 

 this and enclosed in a box was an inclined white screen 

 illuminated by a small glow-lamp. By adjusting the illu- 

 miuation of the lower screen until the star-shaped hole more 

 or less completely disappeared, the illumination of the hori- 

 zontal screen was measured. After trying several different 

 methods, the illumination of the lower screen was adjusted by 

 altering its inclination to the glow-lamp. I greatly prefer 

 this arrangement of screens to a Bunsen Photometer, first, 

 because there is only one spot to examine instead of a pair of 

 images, and secondly, because, under favourable conditions, an 

 almost complete disappearance of this spot may be effected, 

 instead of a similarity between two images, as with a Bunsen 

 spot. A Bunsen screen with which the spot disappears on 

 both sides simultaneously is rare. I understand that by 

 warming the grease-spot the edge may be softened, and that 

 simultaneous disappearance may be secured. I have never 

 seen such a screen, and am inclined to think that the disap- 

 pearance is illusory, and that such softening is probably 

 accompanied by decreased rather than by increased precision. 

 I have recently applied my arrangement of screens to 

 ordinary light-photometry (as distinguished from illumination- 

 photometry). My first plan consisted of two screens (fig. 1)^ 

 each inclined at 45° to the direction of the lights 

 and to the eye. One screen was immediately behind -^S- 1 - 

 the other ; the front screen was perforated, and 

 was mounted on a sliding-carriage on a photometer- 

 bar. The lights were placed, the one a little in 

 front and the other a little behind the plane of 

 intersection of the screens. The back of the 

 perforated screen was blackened and was shaded 

 from the light which illuminated the back screen. 

 The edge of the perforation was bevelled, to assist 

 the complete disappearance of the hole. The hole 

 consisted of two lozenge-shaped apertures one over the other, 

 point to point, the object being to concentrate attention on a 

 vertical line. The screens were held in a frame capable 

 of rotation round a vertical axis through a small angle, 

 for the purpose of producing small and rapid variations. 

 But although one screen thus received more light and the 

 other less, the cosine-law of illumination caused the former 

 to increase but slightly in brightness while the latter dimin- 

 ished considerably. It should be observed that this arrange- 

 ment of screens, although developed from a Bunsen photometer, 

 turns out to be a modification of the Thompson-Starling 



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