388 Intelligence and Miscellaneous Articles. 



light, such as was used in the measurements with the photometer- 

 box, no loss due to this cause was possible. 



In the apparatus devised by Mr. Keeler for an experimental 

 determination of the loss by diffraction, the star was replaced by an 

 illuminated pinhole in the focus of a 3-inch collimating-telescope. 

 This was viewed by an observing-telescope of nearly equal size, in 

 the eye-tube of which was an unsilvered plane-glass mirror, which 

 reflected into the eye-piece a comparison star — the image of an 

 illuminated pinhole produced by a collimating-telescope at right 

 angles to the other two. In the path of the rays from this tele- 

 scope could be interposed the wheel-photometer. The light before 

 entering the first pinhole suffered reflection from an unsilvered 

 glass surface, in order to reduce its intensity to that of the com- 

 parison star. 



The two images in the field of view having been adjusted to 

 equality, the wire-gauze screen was interposed between the object- 

 glasses of the collimating- and observing-telescopes, reducing the 

 light of the star and producing around it the well-known diffraction- 

 image of a network. The wheel-photometer was then introduced, 

 and the intensity of the comparison star reduced until it was equal 

 to the central image of the other. By enlarging the pinholes until 

 the superposition of the colours produced white light, the intensity 

 of the diffraction-images could also be estimated. 



It was thus found that the central image had only -175 of its 

 original brightness, which would therefore be the proportion trans- 

 mitted by the screen under these conditions, and that the brightness 

 of each of the four first spectra was '05 of that originally possessed 

 by the central image. Two thicknesses of the wire-gauze transmitted 

 barely *02, as measured by the intensity of the central image. 



The screens with which these experiments were made were much 

 coarser than the original ones, and it was expected that the effect 

 of diffraction would be less pronounced. The transmission of one 

 thickness, measured by the photometer-box, was *47 ; of two 

 thicknesses. -21. 



Finally, the apertures of the screen and the diameter of the wire 

 were measured by a micrometer-microscope and the apertures found 

 to occupy *465 of the total area of the screen. 



It was concluded, therefore, as the result of the experiments : — 



1. That the transmission, as measured by the photometer-box, 

 was equal to the ratio of the sum of the areas of the apertures of 

 the screen to its total area, and therefore could be considered to be 

 the true transmission of the screen ; and 



2. That the much smaller transmission of the screen, when used 

 in front of the object-glass of a telescope to diminish the apparent 

 brightness of a star, is satisfactorily accounted for by the loss of 

 light caused by diffraction under these circumstances. 



3. That screens used for this purpose should have their constants 

 determined by special experiments of the nature of those just de- 

 tailed, and that their photometric use should then be limited to the 

 reduction of the light of bodies possessing a small angular magnitude. 

 — Silliman's American Journal, September 1885. 



