452 Cambridge Philosophical Society, 



March 23. — Mr. Cowper on the manufacture of lace by machinery. 

 March 30. — Dr. Grant on the metamorphosis of the Amphibia. 

 April 6. — Mr. Faraday on Mr. Ward's plan of preserving and 

 growing plants in closed vessels and places. 



CAMBRIDGE PHILOSOPHICAL SOCIETY. 



A meeting of this Society was held on Monday evening, February 

 26th, the Rev. L. Jenyns, Vice-President, in the chair. Various 

 presents of books were announced, and the following papers were 

 read : — On some new genera of fossil multilocular shells in the slate 

 rocks of Cornwall, by Mr. Ansted, of Jesus College ; On a question 

 in the Theory of Probabilities, by Mr. De Morgan ; On the Quadra- 

 ture of the Circle, by Dr. CressweU. 



A meeting of this Society was held on Monday evening, March 

 12th, the Rev. the Master of Christ's College, the President, being 

 in the chair. Mr. Kelland, of Queen's College, read the first part 

 of a paper On Molecular Attraction. Afterwards Professor Henslow 

 gave an account of the plants brought by Mr. Darwin from the 

 Keeling Islands. These are coral islets of recent formation, lying 

 to the south of Sumatra. They are of the form called lagoon islands, 

 the average height of the land above the water not being more than 

 six feet. These islands have only recently been inhabited by man. 

 The indigenous vegetable species from them are 24 in number, and 

 Mr. Darwin has brought home 22 of these, belonging to 21 genera, 

 and 18 different families. 



March 26th. — The Rev. Dr. Graham, the President, in the chair. 

 Professor Challis read a paper On the Proper Motions of the Stars. 

 Mr. Airy read the termination of a paper " On the Intensity of 

 Light in the neighbourhood of a Caustic," of which the following 

 is an abstract. Taking V to represent the length of the path 

 from a source of light to any point of a reflecting surface (or, 

 mutatis mutandis, of a refracting surface), and thence to a 

 ■point at which the intensity of light is to be estimated, and 

 putting X for the ordinate of the point of the reflecting surface, 



-^^ — - has a finite value at all points, except when the point whose 

 dx 



ordinate is x is the same with the point which, on the ordinary laws 



of reflexion, would reflect light to the point under consideration ; for 



that point , = 0. From this the author deduced that, if the 

 dx 



point under consideration were a conjugate focus, receiving the rays 



reflected from the whole surface, V must be constant, or the whole 



series of differential coefficients must vanish ; but if the point under 



consideration is the focus only for a very small pencil reflected from 



the point whose ordinate is x, and from neighbouring points, then 



d (V) rf2 /y) 



— ^^ — - = 0, and — r— ^ = 0, without any condition for the remam- 

 dx dx'^ ^ 



ing diflferential coefficients. From the nature of the caustic it is 



