316 Dr. G. J. Stoney on the Limits of Vision : 



57 and 59), or 27*3 times as much. If the proportion of in- 

 crease of temperature to that of surface were known, the 

 average magnitude of the particles of the finest insoluble 

 powder might perhaps be calculated. 



The evolution of heat by the mere contact of solids and 

 liquids which do not in the ordinary meaning of the term 

 (i chemically unite," must to a minute extent affect the de- 

 termination of the specific heats of insoluble powders by the 

 method of mixing them with water. And if contraction of 

 volume follows immediately upon its loss of heat, then the 

 specific gravities of insoluble powders when arrived at by the 

 process of weighing them, first in air and then in water, are 

 probably also slightly influenced. 



The heat produced by the mere contact of insoluble solids 

 such as silica, alumina, &c. with water and aqueous solutions 

 of salts, may account for that produced by spring-water, sea- 

 water, mineral-water, &c, filtering through geological strata, 

 and for that developed in other cases of underground tem- 

 perature where ordinary chemical action is absent. 



XXVIII. On the Limits of Vision : ivith special Reference to 

 the Vision of Lnsects. By G. Johnstone Stoney, M.A., 

 D.Sc.j F.R.S., Vice-President, Royal Dublin Society *. 



Contents. 



Fage 



Introductory Remarks 316 



Section I. — Of Vision in general 317 



Section II. — Of Vision with Compound Eyes 325 



Lntroductory Remarks. 



THE President of the British Association, at the recent 

 meeting of that body in Nottingham, mentioned in his 

 opening address that the image formed by the compound eye 

 of an insect had been photographed. This suggests the 

 inquiry how the image is formed, and what is the limit of the 

 vision of which it is the physical basis. The investigation of 

 this point shows that insects cannot see very minute objects, 

 and the whole inquiry seemed of sufficient interest to be laid 

 before the Royal Dublin Society, especially as it suggests 

 much further study which the author could not attempt, but 

 which there are other members of the Society most competent 

 to undertake. 



* From the ' Scientific Proceedings ' of the Eoyal Dublin Society of 

 the 20th December, 1893. Communicated by the Author. 



