RELATIONS TO OTHER SCIENCES 417 



modified by contrast, and preparatory treatment of the retina, are 

 open to all normal-sighted persons. The careful study of these ele- 

 mentary perceptions must furnish an essential stone for the future 

 edifice of mental science. 



The ordinary subjective tests of refraction, which occupy so large 

 a part of the time of every practicing ophthalmologist, furnish data 

 which, carefully selected and arranged, would be of much import- 

 ance in psychology. The response to the simple test of improving 

 or impairing vision by a change of lens shows characteristics constant 

 for the individual, but which vary widely in different persons. The 

 routine which any one adopts in the subjective testing of ametropia 

 furnishes the fairly constant conditions of experiment calculated to 

 best bring out class types and individual peculiarities of reaction. 

 Surely some ophthalmologist interested in this matter will place 

 some of this material at the command of students of mental science. 



Laws of Heredity and Congenital Variation. Attention should 

 be called to the fact that ophthalmology offers an important and 

 promising field for studies of the laws of heredity. I have already 

 referred to the tendency exhibited by the tissues of the eye to ad- 

 here strictly to type, in their development and in their resistance to 

 accidental influences. Already enough has been observed to warrant 

 the supposition that, in the eye, departures from the normal type 

 are themselves apt to be typical. Take the well-known facts regard- 

 ing congenital defects of color perception. The similarity of the 

 disability in enormous numbers of cases, and the tendency to descend 

 to grandsons, through the daughters only, are strongly typical. Such 

 typical instances would seem to promise most for an elementary 

 knowledge of the laws of heredity those laws which have the 

 widest and deepest importance for the sociology of the future. It 

 must be mentioned, however, that this law of descent through the 

 female to the male does not apply universally. We have in ophthal- 

 mology enough groups of exceptions to quite limit and define its 

 scope. 



The range of ophthalmic observations already available in this 

 direction is a wide one. The congenital anomalies of the eye and 

 the individual peculiarities it may present, as to color of iris, pig- 

 mentation of the eyeground, distribution of vessels, and especially 

 anomalies of refraction, as well as the ocular diseases, have been well 

 worked out, and they are capable of comparatively exact notation 

 and record. Statistical studies regarding them, extending over 

 family or race groups, can be relied on as giving facts of definite 

 value. There are already accumulated many observations of great 

 interest in this connection. The reversion to an ancestral type of 

 pigmentation, in retinitis pigmentosa, the striking condition of 

 amaurotic family idiocy, the predisposition of the Hebrew race to 



