384 SIR DAVID BREWSTER ON THE OPTICAL PHENOMENA, NATURE, 



in the morning before the sensibility of the retina had been diminished by ex- 

 posure to daylight, and found that they were neither increased in number or in- 

 tensity. I varied this experiment by diminishing the sensibility of the retina. 

 This was done by holding a bright gas flame close to the eye, and near the axis 

 of vision, till the retina lost its sensibility to all the rays of the spectrum, except 

 a few of the more refrangible ones.* In this case, too, the MUSCCE were as nu- 

 merous and distinct as before, and we may therefore consider it as certain, that 

 the Musav described by Mr Ware, in so far as they were of the same character 

 as those in the healthy eye, are not affected by any variation in the sensibility of 

 the retina. I am disposed to think that they consisted of the ordinary Muscce 

 seen simultaneously with others produced by the pressure of the bloodvessels on 

 the retina, and that it was the latter only which underwent the variations which 

 he describes. 



It is not easy to form any rational conjecture respecting the cause and pur- 

 pose of the numerous filaments by which the Muscce are produced ; for as they 

 exist in all eyes, whether young or old, they are neither the result of disease, nor 

 do they indicate its approach. Were they fixed or regularly distributed, we might 

 regard them as transparent vessels which supply the vitreous humour ; but ex- 

 isting, as they do, in detached and floating portions, they resemble more the 

 remains of some organic structure whose functions are no longer necessary. But 

 though these filaments have no morbid character, they may nevertheless obstruct 

 and even destroy vision. They certainly interfere with nice microscopical observa- 

 tions, and in observing the minute and almost imperceptible lines in the solar spec- 

 trum, I have found them to be occasionally injurious. It is quite possible that some 

 of the cells near the retina and around the optic axis might be filled up with accu- 

 mulated Muscce, and produce a considerable 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 per- 

 sons of a nervous habit or constitution as Muscce wlitantes, and they immediately 

 suppose that they are about to lose their sight by cataract or amaurosis." Pro- 

 fessor 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 distin- 

 guish between the Muscce described above, and those appearances which indicate 

 amaurosis, 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 he 

 has already been the means of freeing from alarm many persons with Musca; 

 wlitantes, and that he had even done this to a distinguished physician. { 



* Lond. and Edin. Phil. Mag., 1832, vol. i., p. 172, vol. ii., p. 168 



t Practical Treatise, &c., p. 751. 



J Professor PLATEAU mentions that he had been led to suppose that the Muscce had their seat in 



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