Mr Ewing on the Eyes of Insects. 297 



which requires us to know, beside the ratio of condensation h 

 when it becomes solid, the law of the condensation and dilata- 

 tion in the body in question in the liquid state, viz. the 

 temperature E of its vaporisation at the ordinary pressure, the 

 depression T of its minimum of temperature below the tem- 

 perature of its ebullition, and the two coefficients g and 2A of 



this law. 



This formula will therefore not differ from that for liquids, 

 taken in its generality, and relatively to their density D', ex- 

 cepting in the presence of the factor V - depending on the 

 condensation or dilatation of the body in its passage from the 

 liquid to the solid state. j 



Art. XVII— On the Structure of the Eyes of Insects. By 

 Mr William Ewing. In a Letter to the Editor. 



Sir, 

 Having diligently employed my leisure time in investigating 

 the structure of the eyes of insects, and conceiving I had 

 made some progress beyond what was at present known on 

 that subject, I arranged my observations into a shape for pub- 

 lication, which I had put into the hands of Dr Hooker to be 

 forwarded for your approbation. That gentleman's knowledge 

 of the subject, however, prevented it, and favoured me with 

 the third volume of a very learned and systematic work, at 

 present publishing on the subject. The impressions which I 

 have received from my observations and experiments differ 

 materially from the descriptions given in that work ; I will, 

 therefore, under the same arrangement, submit to you a few 

 remarks on the Ocular Organs of Insects. 



Simple Eyes. 

 These are the eyes with which we find insects provided in 

 the first state of their existence, as well as those that are pro- 

 duced perfect from the egg. They differ, indeed, as to number, 

 situation, and arrangement, in those insects, but they are 

 identically the same ; their structure is that of a double con- 

 vex lens, but more convex without than they are within : 

 they are all transparent when cleaned, and capable of refract- 



