104 Mr. J. M. Wilson on Diminution of direct Solar 



at other times so black as to eclipse the rings of Newton. Their 

 colour is very variable, sometimes black, with colourless inter- 

 vals, and sometimes richly coloured with the tints of the spectra 

 on which they are seen. When the grating is pressed upon the 

 convex surface, or raised slightly from it, the rings exhibit the 

 same phenomena as those of thin plates. 



When the ray R R' (fig. 19) from the bar of light reaches the 

 eye at E, the grooves being slightly inclined to the plane of re- 

 flexion,- the hyperbolic bands are seen as in fig. 12 ; and when 

 the ray rr' reaches the eye at e, the hyperbolic bands are seen 

 as shown in fig. 13; and when the eye receives all the rays be- 

 tween R' and r', the direct and inverted systems of hyperbolas 

 are seen, as in fig. 20. If, when these are seen, we look at the 

 surface of the grating, we shall see the system of concentric 

 rings produced by the union of the two systems of hyperbolas. 



XIII. Some Remarks on an observation of Mr. Glaisher's. By 

 J. M. Wilson, M.A., Fellow of St. John's College, Cambridge, 

 Mathematical and Natural Science Master in Rugby School. 



To the Editors of the Philosophical Magazine and Journal. 

 Gentlemen, 



A 



T the Meeting of the British Association in Birmingham 

 last September, Mr. Glaisher read in Section A a report 

 of his observations made in balloons during the previous year. 

 One of these was new, I believe, to those who were present, and 

 attracted some attention — Principal Forbes, Mr. De la Rue, and 

 others taking part in a brief discussion about it. No account of 

 this has yet appeared, as far as I know ; and as it may interest 

 the readers of your Magazine, I shall offer you a brief statement 

 of it, and some remarks concerning it. 



That there is always a difference of reading of two thermome- 

 ters, one with blackened bulb exposed to direct sunshine, and the 

 other with bulb concealed, is well known. The latter marks the 

 temperature of the air surrounding it ; the former marks the 

 sum of this and of the direct radiated heat which it receives from 

 the sun. Mr. Glaisher has shown that the difference of reading 

 of the two thermometers, when taken to great heights, instead of 

 increasing or remaining constant, as would seem at first sight to 

 be expected, actually diminishes, and appears as if it would di- 

 minish without limit. If a diagram were constructed in which 

 the abscissae were temperatures and the ordinates heights above 

 the sea-level, the readings of the two thermometers would be in- 

 dicated by two lines, — that of the former by an inclined line 

 nearly straight, indicating a gradual decrease of temperature on 



