402 Cambridge Philosophical Society, 



From the foregoing arguments and facts, it was urged that mo- 

 tions and forces, which certainly exist in cases of combustion, would 

 produce phsenomena exactly similar to thofe of heat, and therefore 

 that part of the pha?nomena usually attributed to heat are due to 

 this motion ; and if part of them, probably the whole. And further, 

 that if the phsenomena of radiation of heat are explained by this 

 motion of the particles of matter, light is simply radiated heat of 

 considerable intensity ; and that imponderable substances, whether 

 under the names of aether, caloric, or phlogiston, are equally ima- 

 ginary. 



Also, a paper was reaS by Mr. Maxwell on the Elementary Theory 

 of Optical Instruments. 



The object of this communication was to show how the magnitude 

 and position of the image of any object seen through an optical in- 

 strument could be ascertained without knowing the construction of 

 the instrument, by means of data derived from two experiments on 

 the instrument. Optical questions are generally treated of with 

 respect to the pencils of rays which pass through the instrument. 

 A pencil is a collection of rays which have passed through one point, 

 and may again do so, by some optical contrivance. Now if we sup- 

 pose all the points of a plane luminous, each will give out a pencil 

 of rays, and that collection of pencils which passes through the in- 

 strument may be treated as a beam of light. In a pencil only one 

 ray passes through any point of space, unless that point be the focus. 

 In a beam, an infinite number of rays, corresponding each to some 

 point in the luminous plane, passes through any point ; and we may, 

 if we choose, treat this collection of rays as a pencil proceeding from 

 that point. Hence the same beam of light may be decomposed into 

 pencils in an infinite variety of ways ; and yet since we regard it as 

 the same collection of rays, we may study its properties as a beam 

 independently of the particular way in which we conceive it analysed 

 into pencils. 



Now in any instrument the incident and emergent beams are com- 

 posed of the same light, and therefore every ray in the incident beam 

 has a corresponding ray in the emergent beam. We do not know 

 their path within the instrument, but before incidence and after 

 emergence they are straight lines, and therefore any two points serve 

 to determine the direction of each. 



Let us suppose the instrument such that it forms an accurate 

 image of a plane object in a given position. Then every ray which 

 passes through a given point of the object before incidence passes 

 through the corresponding point of the image after emergence, and 

 this determines one point of the emergent ray. If at any other 

 distance from the instrument a plane object has an accurate image, 

 then there will be two other corresponding points given in the inci- 

 dent and emergent rays. Hence if we know the points in which an 

 incident ray meets the planes of the two objects, we may find the 

 incident ray by joining the points of the two images corresponding 

 to them. 



It was then shown, that if the image of a plane object be distinct. 



