112 Selentifie Proceedings, Royal Dublin Society. 
In high-power microscopy a minute brilliant focal point is formed 
in such a position that these obsourities, if existing, are well 
perceived; more conveniently a luminous point is obtained by 
illuminating a minute aperture in an opaque soreen held near the 
eye. The shadows thrown on the retina can now be projected 
through the aperture to form an enlarged virtual image on a 
ground-glass screen at some distance from the eye. This is the 
principle of the Entoptiscope devised by the author, and described 
in Part IT. In this way not only ean incipient cataract be detected, 
delineated, and its progress observed, but many other entoptic 
phenomena can be examined with ease. 
The present communication deals with a class of entoptic 
phenomena briefly referred to in Part III., which, though long 
known, I have recently submitted to careful examination. In 
this work I have been assisted by one of my senior students, 
Mr. J. Ledwidge, a.r.cse., who has remarkable facility for per- 
ceiving and depicting these phenomena. Mr, Ledwidge has 
recorded his observations in a Note appended to this paper. 
l. Harprnerr’s Turts. 
In the year 1844 Professor W. Haidinger, the distinguished 
geologist and physicist of Vienna, discovered the curious fact that, 
on looking through a Nicol’s prism at a bright sky, there was 
seen in the direct line of vision a pair of small yellow cones joined 
apex to apex, like an hour-glass.' These the discoverer called 
Biischel, in French houppes, or tufts, and they are now universally 
known as “ Haidinger’s brushes,”’ though fu7?s would be a more 
correct translation and designation. At right angles to these 
tufts a pale blue or violet colour is usually seen filling the larger 
space on each side of the apex of the yellow cones (see Plate VIL, 
fig. 1). The major axis of the yellow tufts coincides with the 
* Poggendoril’s Annalen, 1844, vol. Usiii,, p. 29. 
» Brewster speaks of the tufts as “ brushes,’* “ tufts,'? “ seotors,’? and In 
several places he calls them * bushels **—a term which puzzled me, and doubtless other 
of his readers, until I found it was meant for the German Brsehel. I would suggest 
Jaseicula latea as a more appropriate name. This would have the advantage of & 
verbal association with the _mecade daiea, or yellow spot, with which they are closely 
connected, 
