22 Sir David Brewster on the Optical Figures produced by 



duces by reflexion, we shall see four rectangular radiations, as 

 in fig. 10, each radiation being perpendicular to a side of the 

 square, and consequently passing into one of the three radiations 

 formed by each face of the primitive octohedron. By successive 

 actions, these four radiations become shorter towards the central 

 image, which gradually grows fainter and sometimes disappears. 



If the same experiment is made with the twelve faces formed 

 on the twelve edges of the octohedron, we shall obtain a figure 

 with two radiations, forming an oval line with the image of the 

 candle in the middle of it. This image becomes gradually nebu- 

 lous and finally disappears, leaving a kind of elongated oval 

 nebula, with a dark oval centre, as shown in fig. 11, where the 

 line AB is perpendicular to the replaced edge, and parallel to an 

 axis of the octohedron. The two radiations A, B obviously pass 

 into one of the three radiations given by the adjacent faces of 

 the octohedron ; and if we were to cut a great number of arti- 

 ficial faces variously inclined from that which gives the two radia- 

 tions in fig. 11, to that which gives the /owr in fig. 10, we should 

 observe fig. 11 gradually passing into fig. 7, and acquiring a 

 third radiation, and fig. 7 passing into fig. 10, and acquiring a 

 fourth radiation. 



From the phsenomena exhibited by alum I proceed to those 

 produced by fluor-spar, a mineral having the same primitive form 

 and cleavage. Having immersed one of the faces of the octo- 

 hedi'on for a few days in sulphuric acid, I obtained by reflexion 

 the beautiful figure shown in fig. 12*. The three principal 

 radiations. A, B, C, with the luminous triangular centre, are 

 first developed, and by continuing the action of the acid, six 

 new images are produced at e, f, g, h, and f, k, connected by 

 lines of light with the other part lif the figure. A continuance 

 of the action developes six luminous curves proceeding from the 

 images ef, gh, ik, as in fig. 13, having each a new image within 

 their concavity. Three insulated images appear also at /, m, n, 

 distant 120° from each other, and 60° from the principal radia- 

 tions. 



When the faces of the cube formed by planes replacing the 

 angles of the octohedron are acted upon by the acid, the beautiful 

 figure shown in fig. 14 is produced, the half-moons at the four 

 angles being more distinctly brought out in some cases than in 

 others. 



The mutual connexion of these figures will be seen in fig. 15, 

 where the triangles represent the faces of the octohedral pyramid 

 unfolded as it were, and the quadrangular figure the square base 

 of the pyramid. 



Among the variations of figure produced by th? strength of 

 * See notes on pp. VJ and 28, 



