58 Mr. S. P. Thompson on the Chromatic Aberration 



white paper. These were cut by an assistant and adjusted — 

 their distance and size being unknoAvn to the observer, and 

 varied between each experiment. Hence, under the condi- 

 tions given, the eye could form no judgment of distance 

 based upon apparent magnitude, upon parallax, or upon 

 apparent form or shadow ; the estimation depended solely 

 upon the muscular sensation of adjustment to focus. After a 

 little practice this became moderately easy with white light, 

 and the estimations were fairly within limits. With a screen 

 of yellow glass interposed no difference was experienced. 

 When a ruby-red glass was employed, however, the estimations 

 were generally, though not invariably, too small, and with a 

 cobalt-blue glass generally (though not invariably) too great. 

 The following is the mean of six experiments taken at random 

 from a number made with a red glass, and of another six from 

 those made with a blue glass. 



Eed. Blue. 



Mean real distance 208 centims. 178 centims. 



Mean estimated distance 163*1 ,, 199 „ 



35. When a solution of permanganate was employed in 

 conjunction with a blue glass, giving equal intensities of red 

 and blue rays, the attempt to estimate the distance of the 

 white pieces of paper became perplexing in the extreme. The 

 object seemed to be moving backwards and forwards as the 

 focus changed from red to blue and blue to red. 



36. After about forty experiments with blue and red glasses, 

 the errors became less. Evidently the effect of practice was to 

 sophisticate the result, the mind knowing the probable result 

 beforehand, and making allowance involuntarily. The forty 

 experiments were not all made at one time, because the eye 

 becomes fatigued when experimentation is prolonged ; and as 

 in my own case with fatigue partial astigmatism sets in, I was 

 carefulto test my vision from time to time, to avoid error from 

 this source *. 



37. Every book on Art will tell us that blue is a " retiring" 

 colour, while red is an " advancing colour." For long I had 



* Since the experimental results recounted in this paper have been 

 chiefly obtained with my own eyes, it may he well to state the following 

 particulars: — For objects in centre of field of vision there is no trace 

 of colour-blindness ; neither eye is astigmatic when not fatigued ; their 

 range of accommodation is from 11 centims. to infinity, and with good 

 definition at all ranges ; the blind-spots are distinct, the fovese centrales 

 M^ell formed and sensitive ; there is no difficulty in perceiving Purkinje's 

 figures and Haidinger's brushes and lavender streaks. Let me add, as 

 a test of definition, that on a clear night I can easily see twelve stars of 

 the Pleiades. 



