24 Sir David Brewster on the Optical Figures produced hy 



Having immersed a rhomb of calcareous spar in dilute nitric 

 acid, four parts of water being added to one of acid^ I observed 

 the reflected figure from all the faces of the rhombohedron to 

 have the form shown in fig. 21. The obtuse angle of the crystal 

 was in the direction CE, and the angle ACB was greater than 

 120°. As the obtuse angle of the opposite face has an opposite 

 direction, the figure which it gives by reflexion is the inverse of 

 fig. 21 ; so that, by looking through the parallel faces, we obtain 

 a figure with six luminous radiations. By varying the strength 

 of the acid, the time of its action, and taking the surfaces of 

 diff'erent crystals, the figure undergoes remarkable changes ; but 

 though two individual figures often occur between which no 

 similarity exists, yet, by observing the transitions of a con- 

 siderable number, we may trace the family likeness through 

 them all. 



The thin web of light AEB, BD, and DA, appear at an early 

 stage of the action, but it is often wanting between A and B ; 

 and by continuing the action, a radiation often appears at F, 

 sometimes united, and sometimes not, with the centre C. The 

 radiation CD sometimes expands suddenly below C into a diver- 

 ging brush of light, and in other cases it is often wholly wanting, 

 as well as the triangular luminous centre C. In this case we 

 have only two luminous brushes. A, B, with a small central 

 image at C, A and B being sometimes joined by bright light, 

 and sometimes by a small arch of nebulous light, the centre of 

 which was at C. 



On the faces of three difterent crystals, a figure with five 

 radiations, diverging at unequal angles, was produced. Two of 

 these were the radiations A, B, the third was the brush developed 

 at E, and the fourth and fifth were formed by the division of CD 

 into two branches. Sometimes the whole of the central part of 

 this five-rayed figure was wanting, leaving the expanded part of 

 the radiation in the circumference of a sort of oval ring, which 

 was sometimes luminous throughout, but studded with the five 

 brushes of stronger light. 



When the solvent was pretty strong muriatic acid and water, 

 the figures have often a great similarity to those already described ; 

 but in some cases they have the form of luminous shields of a 

 triangular form, as shown in fig. 22. The place of the central 

 image is at C ; the brightest part of the figure is at E, with a 

 reddish margin, and the next brightest at A and B. In other 

 crystals the lights A, B, E were connected with C by the radia- 

 tions ; and in one case, where a weaker acid was used, E was 

 elevated further above C, and a horizontal band of light passed 

 below C to the sides AD, BD. 



When strong vinegar was used as the solvent, I obtained the 



