Reflexion of Light from Iceland Spar. 



267 



the axes and in the azimuth of the major axis of the ellipfcicilly 

 polarised light, and was observed in the case of two different crystals 

 which were made the subject of experiment. 



Since the light reflected from the surface of the crystal is exceedingly 

 nearly plane polarised, the absolute value of the change in the ratio of 

 the axes is small ; but the relative change is considerable, for tan w is 

 altered by polishing from Q'0334 to 0'0252. Also the change in the 

 azimuth of the major axis is not very large. 



As regards the disturbing causes, it is found that temperature and 

 time do not cause any very perceptible alterations in the surface state 

 of a polished crystal . 



The experiments prove a result unnoticed by Seebeck, that an 

 emery-rouge polished surface gives perfectly concordant results on 

 repolishing, and in this respect is quite as satisfactory as the chalk- 

 polished surface recommended by him. This conclusion is supported 

 both by the elliptic and simple analysers. And in general the results 

 of the paper tend to confirm the views of Seebeck rather than those 

 of Sir J. Conroy, for Seebeck in his paper prefers polished surfaces 

 because of the liability of the natural surface to tarnish. 



In conclusion, my best thanks are due to Mr. Grlazebrook for his 

 advice, and to Professor J. J. Thomson for placing at my disposal a 

 room and apparatus in the Cavendish Laboratory. 



Ci Further Experiments on the Distribution of Micro-organisms 

 in Air (by Hesse's Method)." By Percy F. Fkankland, 

 PhD., B.Sc, F.C.S., and T. G. Hart, A.R.S.M. Communi- 

 cated by Prof. Frankland, D.C.L., F.R.S. Received 

 November 22, — Read December 9, 1886. 



[Plate 3. J 



In a previous communication entitled " The Distribution of Micro- 

 organisms in Air," a number of experiments have been recorded by 

 one of us on the relative abundance of microbes in the air of various 

 places and of the same place at different times. The numerical 

 determination of the aerial micro-organisms in these experiments was 

 made by means of Hesse's apparatus, the method of using which was 

 there fully described. Since the publication of the above experiments 

 we have been extending our investigations by means of this method, 

 and the results which we have obtained form the subject of the 

 present communication. 



In addition to the determination of the number of micro-organisms 



VOL. XLII. X 



