56 Scientific Proceedings, Royal Dublin Society. 
mobility is at first very provoking, but they are much steadier 
when the head is bent over the illuminated sheet of paper lying 
on a table, than when the head is erect and the paper vertical.? 
We owe to Sir David Brewster the first determination of the 
magnitude of these muscee and the first suggestion as to finding 
their distance from the retina.” He used two lights placed at a 
distance from the eye: a double shadow of a particular museca was 
thus thrown on the retina; the lights were then moved closer 
together till the projected images touched. Measuring the 
distances of the lights apart and their distance from the eye, and 
the angle subtended by one of the images of the musca, enabled 
him to deduce its size and distance from the retina. The nodal 
point of the eye was taken by Brewster as 0°524 of an inch 
from the retina, a little less than its true value; the width of 
the musca he found to be 0:0012, (1/820th) of an inch; this 
corresponds to 0:03 mm., and its distance from the retina 
1/85th of an inch, equal to 0°3 mm.; but their distance varies, 
and is often further from the retina than this. 
§ 8. 
When entoptic objects are seen by means of a minute illumi- 
nated aperture, or stenopaic screen, and projected on to a surface 
behind the screen, the magnification of the shadow of the pupil 
1 This is due to the fact that in the former case the musce slowly ascend and 
therefore keep in the line of sight; in the latter they move across and disappear out 
of the line of sight. The range of their motion is limited owing to the fact that the 
vitreous humour is divided up by a kind of cellular structure. Many papers and 
treatises have been written on the subject of the muscz; occasionally the musce 
appear fixed in position; these are considered symptomatic of the beginning of mischief 
in the eye, but as a rule they are indicative only of digestive or other slight bodily 
derangement. In my own case, however, mobile muscz have certainly increased both 
in size and persistency since the development of cataract in my eyes, and are worse in 
the right eye, in which cataract is more advanced. It is curious to note how they 
begin to appear and float into the line of sight when searched for. 
2 On the Optical Phenomena, Nature, and Locality of Musce Volitantes, with Obser- 
vations on the Vision of Objects placed within the Eye, by Sir David Brewster, D.c.L., 
F.R.S., Transactions Royal Society of Edinburgh, vol. xv., p. 874, 1843. Brewster unfor- 
tunately gives no details of his experiment, nor data upon which he founded his 
calculation ; but in an article on Entoptic phenomena in the North British Review 
for November, 1856, I find these data are supplied. The article is evidently by 
Brewster himself, and is one of considerable interest. 
