$91.] Limit of Visibility of the Rays of the Spectrum. 509 



" On the Limit of Visibility of the different Rays of the 

 Spectrum. Preliminary Note." By Captain W. DE W. 

 ABXEY, C.B., R.E., D.C.L., F.R.S. Received April 29, 1891. 



In certain photometric experiments it became necessary to find the 

 limit of visibility of the different parts of the spectrum, and also to 

 ascertain what ratio this limit would bear to some fixed luminosity. 

 It should be borne in mind that this question is totally different from 

 acuteness of vision, which some have confounded with it. The two 

 are independent one of the other, and can scarcely be compared. 



The instrument used in these experiments was similar to that 

 described in the note on the examination of a case of Tobacco 

 Scotoma, &c., but the dimensions were modified : A square tube, 

 3 feet long, had an aperture of 2 inches cut in- its side at 2 feet 

 6 inches from one end, and covered over with ground glass. Within 

 the tube, and close to the ground glass, was a mirror, which reflected 

 the light coming through the ground glass on to the end of the tube, 

 and if the ground glass was illuminated by any light the reflection 

 illuminated a card placed at the end of the tube. The illumination 

 of the card could be viewed through a circular hole at the other 

 nd of the tube, in which was fixed a smaller tube, fitting closely 

 [into the eye. If a colour patch from the spectrum was thrown 

 [on to the ground glass, evidently the card at the end of the 

 tube would be illuminated by the colour used, and its disappearance 

 could be effected by means of rotating sectors closing and opening 

 at will, placed in front of the patch. This simple piece of apparatus 

 iswered its purpose most effectively. 



The first point to ascertain was the ratio of illumination of the card 

 that of the patch thrown on the ground glass. The following 

 ingement was made to effect this. The end of the tube, 

 linst which the card was placed, was removed, and a card with 

 square hole, of f-inch side, was inserted instead. This was 

 jvered on the side away from the tube with a piece of Saxe 

 iper, and when viewed from the outside, and when illuminated 

 the light from the ground glass, showed as a square patch of 

 ;ht. Outside of this, and of double the width, but of the same 

 jight, a mask of black paper, with an oblong aperture, was placed 

 that the illuminated square occupied one-half of the oblong, and 

 le other half showed no white paper. An amyl acetate lamp (O8 

 )f standard candle), placed at a fixed distance from this oblong, and 

 a line with the axis of the tube, illuminated both squares ; but a 

 placed in proper position cast a shadow on the translucent square, 

 lowing only the opaque white half to be illuminated. When the 

 3tors above alluded to were placed in front of the lamp, the two 



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