Improved Dark-adaptometer an Improved Dark-adaptometer*

SOME time ago an instrument was described in this Journal intended for testing dark-adaptation in -normal and sub-normal patients. However, when it was tried to investigate gross disturbances (e.g., retinitis pigmentosa), the need was felt for a wider range of readings. When re-designing the apparatus provisions were also made to change the colour of the test-light if desired in order to make the instrument more versatile. As the general outlay of the apparatus has remained the same except for two relevant items, the reader is referred to the previous paper. In the former design (see previous paper, p. 236, drawing I, "9") a blue glass plate was inserted into the path of rays; as this glass plate could not be easily removed, the light of the stimulus was unvarying blue. The same drawing also showed (see "1 I') how the intensity of the test-light was being reduced by openings of different size drilled into a metal-strip. A -limit of this method was given by the fact that a very fine hole is rather difficult to drill, to keep clean, and to center. In the present design the aforementioned limitations have been overcorne as follows: (a) Changing the colouir cf light: A double-walled, rotating disc (see 9A in this paper) replaces the blue filter (9) of the former design. This disc is similar to that used in ophthalmoscopes for changing lenses, and has six openings. Into five of them filters of different colour can be inserted; the sixth hole remains empty, thus passing -uncoloured light when brought into position. In the photo the empty opening (6) is shown centered. (b) Altering the intensity of light: A disc (see 11A in this paper), similar to the colour-filter disc (9A) takes the place, of the metal strip (11) used in the previous design. The six openings in this disc are all of equal size and accomodate six grey filters of different transmission. These neutral screens may be obtained from Messrs. Ilford but if less accuracy is needed they can be prepared quite easily by exposing and developing photographic film. Their density* is determined by photometry. As very dark screens can be tested only with