Intelligence and Miscellaneous Articles. 327 



THE STAR CHROMATOSCOPE \ AN INSTRUMENT TO EXAMINE AND 

 COMPARE THE RAYS OF THE STARS. BY A. CLAUDET, F.R.S.* 



The scintillation of the stars is probably due to the evolution, in 

 different degrees of swiftness, of the various rays their light emits. 

 These rays seem to divide during their long and rapid course 

 through space, as if they were dispersed by a refractive medium, 

 and we see them following each other in quick succession. The 

 change is so instantaneous that, although we see distinctly the 

 various colours, we cannot judge of the separate lengths of their 

 duration. 



It has occurred to me that if we could increase on the retina the 

 length of the sensation produced by each ray, we should have the 

 better means of examining them separately, of comparing their 

 intensities, and the length of every vibration. 



The retina having the power of retaining the sensation of light 

 during a fraction of time, which has been found to be one-third of a 

 second, one ray succeeds another before the entire sensation is 

 exhausted ; so that, supposing the seven rays reach the eye in 

 the third part of a second, two or more sensations are felt at the 

 same moment, and consequently the perception is mixed and con- 

 fused, although the predominating ray is more conspicuous. But if, 

 while a ray is acting upon one part of the retina, we could bring the 

 next ray on another part, so dividing the sensations, we should be 

 enabled to avoid their confusion, and to compare the colour and 

 duration of each. 



If this were done, and moreover, if we could not only separate 

 the various sensations, but make them travel on the retina during 

 the whole time that each sensation lasts, we should have a still 

 better means of comparison. 



This problem may be solved by transforming the infinitely small 

 spot of the star into a large circle, as can be done with an incan- 

 descent charcoal at the end of a wire rapidly revolving round a cen- 

 tre. It is obvious that if the incandescent charcoal during its revo- 

 lution was evolving successively various rays, we could measure the 

 length and duration of every ray by the angle each would subtend 

 during its course. 



This is precisely what can be done with the light of the star. It 

 can be made to revolve like the incandescent charcoal, and form a 

 complete circle on the retina. 



When we look at a star with a telescope, we see it fixed on one 

 definite part of the field of the glass ; but if with one hand we slightly 

 move the telescope, the image of the star changes its position ; and 

 during that motion, on account of the persistence of sensation on the 

 retina, instead of appearing like a spot, it assumes the shape of a 

 continued line. Now if, instead of moving the telescope in straight 

 line, we endeavour to move it in a circular direction, the star appears 



* Communicated by the Author, having been read at the British Asso- 

 ciation Meeting, Newcastle-upon-Tyne, August 28, 1863. 



