327 



might possibly be rendered suitable for such. After a few preiinii- 

 narj trials I had an instrument made for me, differing in many 

 respects from the original one. 



The Nernstlamp was discarded and was replaced by a lamp of 

 greater intrinsic brilliancy. The arrangement of (he illumination-tube 

 was slightly changed so as to allow a relatively greater part of the 

 light reaching the eye. 



In the Zeiss instrument the image of the Nernst filament is pro- 

 jected upon a slit and by means of a second condensor into the 

 pupil of the eye. The light, after leaving the second condensor is 

 deflected by a glass plate, making an angle of 45° with the axis of 

 the tube. The optical system for viewing the fundus looks through 

 this glass plate. With this construction about 8.5 7„ of the light 

 leaving the second condensor is projected into the eye and 9J .5 "/„ 

 of the light leaving the eye reaches the objective of the viewing- 

 tube. 1 placed the glass-plate so as to make an angle of 65° with 

 both tubes, which allowed about 21 "/„ of the light to enter the 

 eye and 79 7o ^o reach the observing eye. In this way the amount 

 of light falling on the photographic plate was about doubled. 



The Nernstlamp has an intrinsic luminosity which I measured as 

 of 3.1 Hefnercandles per square millimeter. By the use of a special- 

 ly constructed halfwattlamp of low voltage I got an intrinsic bril- 

 liancy of nearly 29 units per square millimeter. A suitable small 

 camera having been adapted to the instrument I got, after a few 

 failures, usable negatives of a diameter of 26 to 30 millimeters, 

 showing about 27 degrees of the fundus and covering an area of 

 4V2 times the diameter of a normal optic disc. 



The negatives were sometimes good, though very often blurred, 

 owing to the long exposure of 0.4 to 0.5 of a second. I have tried 

 to get better results with a small arclamp of about 5 amperes but 

 without much success. Though the intrinsic intensity was 3 times 

 greater, the area was notably smaller. With an entirely modified 

 construction and with an arclamp of 25 — 30 amperes better results 

 might be expected as the exposure might have been reduced to 

 7g of a second. As the angle of view was also rather small and 

 could only be enlarged by a complete reconstruction of the appa- 

 ratus, I have kept the instrument as it was, and have tried to get 

 more satisfactory negatives in quite another way. 



With the indirect ophthalmoscopy we can entirely eliminate the 

 reflexes on the cornea and the lens by following Gullstrand's method. 

 But we always i-etain two i-eflexes on the oi)hthalmoscope lens. 

 These do not hin'der visual observation as they are rather small and 



