Proceedings of the British Association. 333 



this viev/. The result obtained by him tended to confirm it very 

 unequivocally ; and so did another series, made the same year in 

 Dolcoath mine, by John Rule, Jr. one of the superintendents. 

 Many other individuals have since at the request of Mr. P., carri- 

 ed on similar observations in different mines, all showing that the 

 subterranean temperature increases in some proportion to the 

 depth of the stratum. The tables of observations given in the 

 Report confirm Mr. Fox's previous views, that the rate of increase 

 is not so considerable at deeper excavations as at those which 

 are shallower. — Mr. Eaton Hodgkinson read a paper on the tem- 

 perature of the earth in the deep mines near Manchester. — Prof. 

 Forbes made his report on the temperature and conducting pow- 

 ers of different strata. The results agree substantially with those 

 reported by him last year. 



Sir D. Brewster read a report on the Phenomena and cause of 

 tnusccB volitantes. As this paper was illustrated with several draw- 

 ings, and contained minute experimental details, it is not easy to 

 give a popular account of it. The following are the principal re- 

 sults. 1. In persons of all ages, and with the most perfect eyes, 

 transparent filaments or tubes exist in the vitreous humor, and at 

 different distances from the retina. 2. These filaments float in 

 the vitreous humor, moving about with the motion of the head. 

 3. These filaments are seen by means of their shadows on the 

 retina, and are most distinctly visible in divergent light, their 

 shadows being bounded by fringes produced by diffraction or in- 

 flexion. 4. The real muscae, resembling flies, are knots tied, as 

 it were, on these filaments, and arising from sudden jerks or mo- 

 tions of the head, which cause the long floating filaments to 

 overlap and run into knots. 5. By making experiments with the 

 head in all positions, and determining the limits of the motions 

 of the muscse, by measuring their apparent magnitude, and pro- 

 ducing double images of them by means of two centres of di- 

 vergent light, the author was able to determine their exact place 

 in the vitreous humor, and to ascertain the important fact that 

 the vitreous humor in the living human eye is contained in cells 

 of limited magnitude, v/hich" prevent any bodies which they con- 

 lain from passing into any of the adjacent cells. The author 

 concluded with the following observations : " I have dwelt thus 

 long on the subject of muscm volitantes, not only because it is an 

 entirely new one, but also on account of its practical utility. Mr. 



Vol. XL, No. 2.— Jan.-March, 1841. 43 



