324 Lord Rayleigh, On the invisibility of small objects. [Feb. 12, 



When the mean radius of a coil has been determined in this 

 manner by comparison with another of proportions more favourable 

 for calculation from the data of construction, other quantities 

 relating to the coil may be deduced by mere calculation. For 

 instance, the important constant g v denoting the mean area 

 included by the windings, is connected with the mean radius A by 

 the equation 



g x = -irA* + -&tt£ 2 . 



A more direct process for determining g 1 electrically is given 

 by Maxwell § 754, and has recently received an important applica- 

 tion in the hands of Kohlrausch. In this method the quantity 

 sought is proportional to the cube of a distance not very easy of 

 precise measurement ; and it is possible that the less direct 

 method explained above may be the more accurate in practice. 



(3) On the invisibility of small objects in a bad UgJit. By Lord 

 Rayleigh. 



In a former communication to the Society (March 6, 1882) I 

 made some remarks upon the extraordinary influence of apparent 

 magnitude upon the visibility of objects whose ' apparent bright- 

 ness ' was given, and I hazarded the suggestion that in consequence 

 of aberration (attending the large aperture of the pupil called into 

 operation in a bad light) the focussing might be defective. Further 

 experiment has proved that in my own case at any rate much of 

 the effect is attributable to an even simpler cause. I have found 

 that in a nearly dark room I am distinctly short-sighted. With 

 concave spectacles of 36" negative focus my vision is rendered much 

 sharper, and is attended with increased binocular effect. On a dark 

 night small stars are much more evident with the aid of the spec- 

 tacles than without them. 



In a moderately good light I can detect no signs of short- 

 sightedness. In trying to read large print at a distance I succeeded 

 rather better without the glasses than with them. It seems there- 

 fore that the effect is not to be regarded as merely an aggravation 

 of permanent short-sightedness by increase of aperture. 



The use of spectacles does not however put the small and the 

 large objects on a level of brightness when seen in a bad light, 

 and the outstanding difference may still be plausibly attributed to 

 aberration. 



