10 Sir David Brewster 07i Miiscae volitantes. 



able degree of blindness; but this is an effect of them which 

 there is little occasion to apprehend. 



Mr. Mackenzie* informs us, "that few symptoms prove so 

 alarming to persons of a nervous habit or constitution as 

 Musca volitantes, and they immediately suppose that they are 

 about to lose their sight by cataract or amaurosis." Professor 

 Plateau of Ghent, to whom I had communicated, at his own 

 request, some of the preceding results, mentions to me, that 

 few physicians are able to distinguish between the Mnscce de- 

 scribed above, and those appearances which indicate amauro- 

 sis, and that they often, without cause, alarm patients who 

 consult them for the first time respecting such affections of the 

 eye. He assures me that the results contained in this paper 

 have already been the means of freeing from alarm many per- 

 sons with Mnscce vulitniites, and that they had even done this 

 to a distinguished physician -j-. 



The details in the preceding pages maj', therefore, be con- 

 sidered as establishing the in^porlant fact, that Mnscce volitantes 

 have no connexion whatever either with Cataract or Amau- 

 rosis, and that they are nearly altogether harmless. This 

 result has been deduced by the aid ot a recondite property of 

 divergent light, which has only been developed in our own day, 

 and which seems to have no bearing whatever of an utilitarian 

 chaiacter. And this is but one of numerous proofs which the 

 progress of knowledge is daily accumulating, that the most 

 abstract and apparently transcendental truths in physical 

 science will sooner or later add their tribute to supply human 

 wants, and alleviate human sufferings, Nor has science per- 

 formed one of ihe least important of her functions when she 

 enables us, either in our own case, or in that of others, to 

 dispel those anxieties and fears which are the necessary off- 

 spring of ignorance and error. 



St. Leonard's Colleire, St. Andrews, 

 March 4, 184.3. 



Postscript. — The Mnscce described in the preceding j)aper 

 increased slightly during the years ISi'S, 184'4', and 184'5, in 

 consequence of the overlapping of the filaments ABDE and 



• Practical Treatise, Sec, p. 7«t1> 



f Professor Plateau mentions in his letter to myself, that he had been 

 led to suppose tliat the MiisccB hixd their seat in tlie vitreous humour ratlier 

 tlian in the aqueous; but that he hail been stopped by the difficulty of re- 

 conciling this opinion with the viscosity of the vitreous humour. As the 

 vitreous humour is pcrfcct't/ Jluid within each cell, the viscosity here sup- 

 posed, being only apparent, no longer presents any difficulty. 



