318 Mr. W. Le Conte Stevens on 



7. Voluntary Control of the Focal Adjustment. 

 Experiments in stereoscopy sufficiently show the office of 

 the rectus muscles in modifying the interpretation put upon 

 retinal sensations. If the visual lines be kept as nearly 

 parallel as possible, and the observer's distance from an object 

 regarded be kept constant, the retinal sensation may still be 

 modified by direct voluntary contraction of the ciliary muscle. 

 It is accompanied by contraction of the pupillary opening ; 

 and this can be approximately measured by an assistant. The 

 experiment is straining, but can be performed with very little 

 associated contraction of the rectus muscles. Within two 

 seconds of time the pupillary opening of my eye has thus been 

 often changed to one tenth of its previous area, the contrac- 

 tion of the ii'is quickly following that of the ciliary muscle. 

 The visual effect is first a slight enlargement of the retinal 

 image as it grows dim ; this is due to the production of diffu- 

 sion-circles, which encroach upon the surrounding retinal area 

 as well as upon the image. The object then apparently dimi- 

 nishes in size to a marked extent. As its absolute distance 

 and diameter, however, are constant, the retinal image cannot 

 be diminished except by encroachment of diffusion-circles. 

 The apparent diminution is far greater than can be explained 

 on this ground, and can only be referred to an error in judg- 

 ment induced by abnormal muscular conditions that have been 

 voluntarily imposed. The experiment has been many times 

 repeated ; and the uniformity of results leaves no room to attri- 

 bute the illusion to accidental circumstances. The apparent 

 distance of the object to which the gaze is directed becomes 

 indeterminate. Ciliary contraction suggests the idea of its 

 nearness ; but the dimness resulting from imperfect focaliza- 

 tion suggests its remoteness. The impression of its contrac- 

 tion in area is unmistakable and striking. Since the con- 

 tractions of the ciliary and internal rectus muscles are usually 

 associated in normal vision, the considerations just expressed 

 explain why so acute an observer as Wheatstone* should have 

 noticed the apparent decrease in size of the image when strong 

 convergence of visual lines was produced by pulling forward 

 the arms of his reflecting stereoscope, but failed to note the 

 corresponding variation in apparent distance ; though he ob- 

 serves that it seems changed in position, but does not say 

 whether the change is that of increase or decrease of distance. 



8. Effect of varying the Locality of the Image. 

 When strong muscular tension is voluntarily induced, the 

 modification of retinal sensation seems to be greatest at the 

 * Philosophical Magazine, 1852, pp. 507 & 508. 



