74 HERMANN VON HELMHOLTZ 



almost ludicrous that I and others should have been so slow 

 as not to see it. It is, namely, a combination of glasses, by 

 means of which it is possible to illuminate the dark back- 

 ground of the eye, through the pupil, without employing any 

 dazzling light, and to obtain a view of all the elements of the 

 retina at once, more exactly than one can see the external 

 parts of the eye without magnification, because the transparent 

 media of the eye act like a lens with a magnifying power of 

 twenty. The blood-vessels are displayed in the neatest way, 

 with the branching arteries and veins, the entrance of the 

 optic nerve into the eye, &c. Till now a whole series of most 

 important eye-diseases, known collectively as black cataract, 

 have been terra incognita, because the changes in the eye were 

 practically unknown, both during life, and, generally speaking, 

 after death. My discovery makes the minute investigation 

 of the internal structures of the eye a possibility. I have 

 announced this very precious egg of Columbus to the Physical 

 Society at Berlin, as my property, and am now having 

 an improved and more convenient instrument constructed to 

 replace my pasteboard affair. I shall examine as many patients 

 as possible with the chief oculist here, and then publish the 

 matter/ 



The ophthalmoscope was, however, some time in making its 

 way, on account of the mathematical and physical knowledge 

 presupposed by the ' Description of an Ophthalmoscope for the 

 Investigation of the Retina in the Living Eye', published in 

 the autumn of 1851, and people were at first very shy of 

 employing it. One distinguished surgical colleague told Helm- 

 holtz he should never use the instrument it would be too 

 dangerous to admit the naked light into a diseased eye; 

 another was of opinion that the mirror might be of service 

 to oculists with defective eyesight he himself had good eyes 

 and wanted none of it. But by December 16 of the same year 

 Helmholtz was able to write to his father : 



' Eighteen orders for the ophthalmoscope have dropped in, 

 one after the other, so that my mechanician is doing a good 

 trade. The world is getting to hear of it.' 



Forty years later he tells the story of its discovery : 



'While preparing my lectures I hit upon the invention of 

 the ophthalmoscope, and then on the method of measuring the 



