IdO 



THE AMEItlCAK MON'THLY 



[October, 



paper, read before the American As- 

 sociation for the Advancement of 

 Science, and noticed in the Septem- 

 ber number of this Journal. 



It is well known that the distance 

 of easiest vision is variable, during 

 the life of the same individual. The 

 " near-point " for a normal eye, varies 

 from three inches for a child of three 

 years, to eighteen or twenty feet for 

 a man of eighty, the power of accom- 

 modation diminishing with increase 

 of age. For such an eye, when in a 

 relaxed state, parallel rays will be 

 converged to the exact distance of 

 the retina. If the radiant point be 

 but ten inches distant, the sheaf of 

 divergent rays from it, if transmitted 

 through the same refracting medium, 

 would be focalized behind the retina, 

 were there not an instant contraction 

 of the ciliary muscle, resulting in an 

 increase of convexity of the crystal- 

 line lens at its front surface. The 

 ease or difficulty with which this is 

 done, depends mainly on the age of 

 the person, if the eye be normal. The 

 effort exerted by a little child will be 

 far less than that of an old man. 



All that the microscope can do is 

 to increase the visual angle under 

 which the object is seen, and hence 

 increase the size of the retinal image. 

 The extent to which this may be 

 advisable depends upon several con- 

 siderations well known to micro- 

 scopists. Since the visual angle is 

 simply the measure of the difference 

 of direction between two rays passing 

 axially through the crystalline lens, 

 from opposite marginal points of the 

 magnified image as seen through the 

 eye-piece, it is quite possible for this 

 to remain sensibly constant, while 

 the refracting power of the crystal- 

 line lens varies. The adjustment of 

 the eye-piece, or the distance of the 

 eye from it, may vary while distinct 

 vision is retained, the limits of varia- 

 tion depending upon the power of 

 accommodation in the eye of the ob- 

 server. For a hypermetropic eye, 

 the rays from a given crossing point 

 near the focus of the eye-piece may 



emerge from the latter either parallel, 

 or slightly convergent, or divergent, 

 and yet be distinctly focalized on the 

 retina in consequence of appropriate 

 action of the ciliary muscle. 



The interpretation which we put 

 upon a retinal sensation is quite un- 

 conscious, and always accompanied 

 with equally unconscious interpreta- 

 tions of attendant muscular sensa- 

 tions. The experience of the individ- 

 ual is the only guide in reaching 

 visual judgments. It is not at all 

 remarkable that different persons 

 should vary in the interpretation 

 they put upon sensations produced 

 under the same external conditions, 

 although the general effect of con- 

 trolling the condition of the eye 

 among them may be much the same. 

 The writer has elsewhere detailed 

 numerous experiments on this sub- 

 ject. (See American Journal of 

 Science, November and December, 

 1 88 1, April and May, 1882.) The 

 result may be briefly stated by say- 

 ing that, while the visual angle re- 

 mains constant, an increase in the 

 contraction of the ciliary muscle, or 

 of the internal rectus muscles if both 

 eyes be employed, produces the 

 illusion that the object is much 

 smaller and nearer ; under such con- 

 ditions, the apparent diminution in 

 size, together with imperfect focaliza- 

 tion, may produce as a secondary 

 effect the illusion that the visual 

 angle has been diminished, and the 

 imagination that the object is more 

 distant. Thus the unmistakable il- 

 lusion is that of diminution of size, 

 and this may be coupled with great 

 lack of determination in the judg- 

 ment of distance. Upon the writer 

 the most usual effect is that of diminu- 

 tion of die^tance. 



The internal rectus and ciliary 

 muscles are supplied from the same 

 nerve, and their contractions are 

 usually simultaneous, though dis- 

 association to a limited extent is by 

 no means impossible. The relaxa- 

 tion of these muscles, with contrac- 

 tion of the external rectus, produces 



