by Wire Gauze Screens. 211 



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 com- 

 parison 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 telescope 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 comparison star. 



The two images in the field of view baving 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 photo- 

 meter 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 colors 

 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 

 transmitted 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 in- 

 tensity 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 trans- 

 mission 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 aper- 

 tures 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 con- 



