Barrett—On Entoptic Vision. 67 
the shadow which the patient has made, the oculist at once knows 
(see § 3) the exact area of the entoptic obscurity, or of the pupil, as 
shown by the dotted circles in fig. 5. A supply of tracing-paper, 
properly ruled and cut the right size, is provided with the 
instrument.? 
GSSE DPan 
Fig. 5 (reduced one half). 
§ 3. 
It is certainly astonishing that such an instrument as the fore- 
going does not appear to have been devised hitherto. Perhaps no 
instrumental appliance was thought necessary for entoptic obser- 
vation, which has fallen into disuse since Helmholtz’s discovery 
of the ophthalmoscope. In fact, Donders remarks: ‘“‘ Now that 
Helmholtz’s ophthalmoscope is in our hands, the importance of the 
entoptic mode of examination for diagnosis is thrown completely 
into the shade.’ Withall deference, I venture to think Donders’ 
statement will be found to be inaccurate. It is a remarkable fact 
that neither Donders nor Helmholtz depicts the entoptic observa- 
tion of cataract, nor dwells upon the special value of this method 
in the diagnosis of this defect in vision. Drawings are given of 
what they and their pupils saw ; but these are only of minor defects 
in the eye. Even in the lengthy article on Ophthalmology in the 
Encyclopedia Britannica and in several text-books on this subject, 
I can find no reference to entoptic observation. It is obvious that 
whilst the ophthalmoscope has been of inestimable value to the 
1 The Entoptiscope can be obtained from the well-known opticians, Messrs. Curry 
and Paxton, 195 Great Portland Street, London, W. 
* Donders, Accommodation and Refraction of the Eye, p. 204. 
