HERMANN VON HELMHOLTZ 



least inexactitude of refraction or of accommodation 

 will allow some of the rays to reach the observer, and 

 we are most likely to succeed if the subject of observa- 

 tion looks to the side. The illumination is most 

 brilliant when the incident light falls on the place of 

 entrance of the optic nerve, because the white sub- 

 stance of the papilla reflects light strongly, and be- 

 cause, on account of its diaphanous structure, the 

 surface is not so uniform as to receive a perfectly sharp 

 image. 



Thus it will be seen that the invention of the 

 ophthalmoscope was not a happy guess, but was the 

 outcome of a careful sifting of the facts of vision. 

 In this invention, and in its theoretical explanation, we 

 have an excellent example of Helmholtz's thorough- 

 ness. In many of its arrangements the ophthal- 

 moscope has been made more easy to use, and the 

 observation of the inverted image has its advantages, 

 but we have it on the authority of Donders and 

 von Jaeger, that the instrument in its original form 

 is optically complete. Helmholtz also saw how the 

 instrument could be serviceable to ophthalmologists, 

 not only as regards the examination of the fundus and 

 the observation of changes in the retina, but also as to 

 how the refractive conditions can be accurately esti- 

 mated. In this way, the degrees of myopia (short- 

 sightedness), hypermetropia (far-sightedness), and of 

 that peculiar condition known as astigmatism (in 

 which, the meridians of curvature of the refractive 

 82 



