I873J 



Physics. 



547 



Mr. J. W. Stephenson, F.R.A.S., and Mr. Charles Stewart, F.R.M.S., make 

 use of the appearances presented by objects immersed in media of different 

 refractive power to determine some points in their structure.* This especially 

 applies to colourless transparent organisms such as the skeletons of diatoms 

 and siliceous and calcareous spicules of sponges. The siliceous deposits, both 

 of plants and animals, are of less refractive index than Canada balsam ; con- 

 sequently, when mounted in that medium they appear, if convex, to act as 

 concave lenses do in air, and vice versa. If diatoms are examined in air, i.e., 

 dry, they are in some instances too opaque for transmitted light, but on im- 

 mersing them in water, of which the mean index is i-366,«they become more 

 translucent ; with media of higher refractive power, the translucency increases 

 until the mean index of strong sulphuric acid (i'434) is attained, in which they 

 become practically invisible. As every object which is transparent and colour- 

 less becomes absolutely invisible when immersed in a colourless medium 

 identical in refractive power with itself, we know approximately that the 

 refractive index of diatomaceous silex is i'434 (much below that of quartz), 

 and this is accordingly for diatoms our neutral point. By progressively 

 increasing the refractive power of the mounting medium the diatoms again 

 become more and more visible, until, as we all know, when mounted in 

 Canada balsam (1*540) the coarser species are sufficiently defined for all ordi- 

 nary purposes ; but if we require a still greater departure from the neutral 

 point, or invisible condition, we must select some other substance of still 

 higher refractive power. This we find in bisulphide of carbon, the index of 

 which is i'678, and by dissolving phosphorus in the bisulphide we may obtain 

 any power between r678 and 2 # 254 ; but when large thick diatoms, such as 

 Hcliopclta, are mounted in a strong solution of phosphorus, they again become 

 nearly, if not quite, as opaque as they were in air. From the above it is 

 evident that, on examination of a diatom or other object in air and in bisul- 

 phide of carbon, they are seen under conditions in which the respective optical 

 effects arising from their form are reversed. The results of the examination 

 of some diatoms are given in the paper, with figures showing the various effects 

 produced by the use of varied mounting media. It is also suggested that 

 some animal tissues, in which the staining process has failed to reveal differ- 

 ences of structure, may be profitably examined in media of such high refractive 

 index as bisulphide of carbon or oil of cassia. 



Lecture Illustrations of Solar Phenomena. — In a recent lecture on " Sunlight 

 and its Source," President Morton, of the Stevens Institute of Technology, 

 employed several new illustrations of his own device, which will interest 

 some of our readers. In the first instance, to illustrate the motion of sun 



Fig. 4. 



^W 



spots across the solar disc, their foreshortening when near the limb, &c, an 

 apparatus was employed consisting of a glass cylinder, on which a sun spot 



* Monthly Microscopical Journal, vol. x., p. 1. 



