718 PHILOSOPHICAL TRANSACTIONS. [aNNO I763. 



and very elastic, not jointed by rings as in lobsters, and on each side of the back 

 there are 2 considerable sharp edged risings, of a black and long substance. Be- 

 tween each eye and the breast, there is a cavity somewhat like the inside of a 

 human ear; but it does not penetrate to the inside. From each shoulder pro- 

 ceeds a strong muscular fin, close by which, towards the breast, is an opening, 

 through which one may thrust his hand and arm quite up through the mouth; 

 and between these fins proceed from the breast 2 short paws, somewhat like the 

 fore half of a human foot, with 5 toes joined together, having the appearance of 

 nails. Near the tail are 2 large fins, one on the back, the other under the belly. 

 The skin is of a dark brown colour, but darker spotted in several places, and 

 entirely without scales. 



XXXI. Rules and Examples for Limiting the Cases in which the Rays of Re- 

 fracted Light may be Reunited into a Colourless Pencil. By P. Murdoch^ 



M.A.,F.R.S. p. 173. 



Let so, pi. 17 J %• Ij a small pencil of the solar light, pass through the re- 

 fracting medium abcd, whose opposite surfaces, represented by ab, cd, are pa- 

 rallel planes: then the violet rays ov, will, in the 2d refraction into the air, 

 emerge parallel to the red; for both will be parallel to the incident ray so, and 

 consequently to each other; that is, yv will be parallel to Rr, as is plain from the 

 common principles of optics. If the light after its emergence be received on a 

 screen, placed any where beyond rv, it will be tinged with violet on the side vf, 

 and with red towards Rr : and if the incident pencil so be exceedingly small, all 

 the intermediate colours will be seen in the same order as when light is refracted 

 by a prism. But if the incident pencil is not very small, or if the luminous body, 

 from which the rays are transmitted through a small opening at o, has a consi- 

 derable breadth, like that of the sun's disk; then so many rays of every kind will 

 mix towards the middle of the spectrum as to produce a pure white; but at the ex- 

 tremities vr and Rr, it will still be tinged with violet and red; for a violet ray from 

 the uppermost point of the sun's disk will be more refracted than the other can 

 be; and a red ray from the lowest part of his disk, will be less refracted than any 

 other. If BC, the distance of the refracting surfaces, be increased or diminished, 

 RV, the distance of the extreme rays, will be increased or diminished in the same 

 proportion; and if RV approach very near to the aperture o, the colours will be- 

 come imperceptible. 



To reunite these rays, we may place another medium of the same refractive 

 power, and of the same thickness {be = bc) as in the figure; so as the rays vv, 

 Br, &c. may enter its surface cd at the same angle as they emerged at from cd, 

 or as so entered ab ; and after refraction at the point o of the surface ab, to 

 which they converge, they will be reunited into os the continuation of so, in a 



