PROFESSOR AT KONIGSBERG 77 



placed at one side of the observing eye is reflected. A portion 

 of this light will then enter the subject's eye, and illuminate 

 its fundus; the rays of light reflected from the illuminated 

 background on to the glass plates will now return partially 

 from the surface of these to the source of light, but another 

 part of the reflected rays will pass through the glass plates, 

 and reach the eye of the observer. But the visual field of 

 the latter, limited by the pupil of the observed eye, is so small, 

 on account of the relatively considerable distance of one eye 

 from the other, that it would be impossible to combine the 

 observed details into a whole ; it is therefore imperative to bring 

 the two eyes as close to one another as possible. This causes 

 the image as a rule to fall behind the observer, who cannot 

 distinguish it plainly, and since a normal eye can only combine 

 parallel and diverging rays upon its retina, and not those that 

 are convergent, Helmholtz (as the readiest means of making 

 the converging beams diverge) inserted a concave lens between 

 the mirror and the eye of the observer, and by this simple 

 contrivance provided the essentials of his ophthalmoscope. 

 He also suggested a number of other practical methods of 

 constructing the instrument. 



.'The construction of the ophthalmoscope,* says Helmholtz 

 at a later time, in discussing his medical and physiological 

 work, 'was a turning-point in my position in the eyes of the 

 world. From that moment I found favour with authorities 

 and colleagues, and was left free to follow the promptings 

 of my scientific curiosity. I attribute my subsequent success 

 to the fact that circumstances had fortunately planted me with 

 some knowledge of geometry and training in physics among 

 the doctors, where physiology presented a virgin soil of the 

 utmost fertility, while on the other hand I was led by my 

 acquaintance with the phenomena of life to problems and 

 points of view that are beyond the scope of pure mathe- 

 matics and physics/ 



The invention of the ophthalmoscope, with the mechanical 

 constructions and modifications which it entailed, as well as 

 a series of experiments in physiological optics undertaken 

 with the instrument, took up no more than the early weeks 

 of 1851. Helmholtz discovered among other points that light 

 which impinges directly on the optic nerve does not give rise 



