Barrett—On Entoptie Vision. 79 
(6). But more important is the study of the structure of the 
crystalline lens as seen in the Entoptiscope. The observer will 
probably notice a star-like figure radiating from the centre of the 
field of view. This is due to slight differences in refractive power 
of the fibrous tissue separating the sectors into which the crystalline 
is known to be divided. This star-figure is sometimes bright, as 
shown in Plate IV., fig. 5, sometimes dark, the difference being due 
to the fact that the refraction of the fibres is in some eyes greater, 
and in some less, than that of the surrounding portion of the 
crystalline. In his wonderful paper on the “‘ Mechanism of the 
Hye,” published in the Philosophical Transactions upwards of a 
hundred years ago, Dr. Thomas Young gave a drawing of the 
radiating structure of the human crystalline, exhibiting ten main 
radiations, and finer striz within.! These finer radiations are well 
seen in the crystalline as revealed by the Entoptiscope. Donders 
and Helmholtz both give several drawings of the lens seen by its 
entoptic image. Bright spots, due to higher refrangibility, are 
frequently seen on the crystalline and are shown in Plate IV., 
fig. 5; these are termed “pearl-spots,’ and, in some cases, 
similar dark spherules are seen, due to the lower refrangibility of 
their structure.’ 
The eminent oculist, Mr. J. Tweedy, President of the College 
of Surgeons, London, published in the Lancet for December, 1871, 
p- 776, drawings of the human crystalline, showing its stellate 
structure, which he was the first to observe in the eyes of several 
patients by means of intense oblique illumination. Mr. Tweedy 
was good enough to show me his original drawings which are 
more detailed than the published reproduction. 
1 On the Mechanism of the Eye, by Thomas Young, M.D., F.k.s., Phil. Trans., 1801. 
This classical research was published when Young was only twenty-eight years old, 
and contains a number of remarkable discoveries, such as accommodation being due to 
change in curvature of the crystalline, &c., often attributed to later investigators. No 
one, however, appears to have noticed that Young was the first to discover entoptic 
vision. He gives excellent drawings (figs. 32 and 33 in his paper) of the radiating 
structure of the crystalline and the mottled surface of the cornea when rubbed, as seen 
when ‘‘a minute lucid point such as the image of a candle in a small concave speculum 
was held very near the eye.” 
* A magnifying glass is required to see them in Plate IV., fig. 5, which has been 
reduced from a drawing made by Donders. 
M 2 
