the Two Rays in Calcareous Spar. 211^ 



ges farther asunder than any one I have met with ; and I do 

 not even except the parallelopiped, prepared according to Wol- 

 laston's directions from two prisms of calcareous spar in which 

 the chief sections 'are made to cross at right angles.* NicoPs 

 instrument not only answers in place of tourmaline, the green 

 variety of which, as is well known, is difficult to be obtained ; 

 but it actually surpasses it essentially, in a property so im- 

 portant in many investigations, that of exhibiting the colour phe- 

 nomena of crystals and other bodies in polarised light in a 

 manner perfectly pure, and free from foreign tints of colour. 



The construction of this instrument will be most easily un- 

 derstood, by reference to the accompanying figure, which re- 

 presents the instrument as sent from England, and for the use 

 of which I have to thank the politeness of Professor Dove. A 

 section of it is here given of the natural size, p 



ab, dh\ is the principal section of a rhomboid of H^ 

 calcareous spar, which has received the dimensions i 

 here represented by cleavage. The long edges, 

 a b' and b a' are the natural obtuse edges of the P^ 

 rhopiboid ; while the terminal planes a b and a'V, are ^ 

 so cut as to form an angle of 68° at a and a! ; whereas, in 

 their natural state, the angle is 71°. The rhomboid thus mo- 

 dified, is cut through in the direction of the line b b\ per- 

 pendicular to the principal section and to the terminal planes 

 a b and a'b' ; and after both the new surfaces have been polished, 

 they are again united by Canada balsam. Both the prismatic 

 halves of the rhomboid-f* can also, as is more easily effected, 

 be cut in the necessary form from two different pieces, taken 

 either from one or from two crystals. The whole is inserted in 

 the cork PPPP, which has been cut through and again united. 



If we look at an object through this rhomboid placed longi- 

 tudinally, parallel to the edges ab' and ba\ we see only one 

 image in this direction, and that the ordinary one ; the extraor- 

 dinary image comes first into view when we incline very much 



• A similar construction for quartz, described and figured in Herschers 

 Treatise on Light. 



+ Accurately speaking, the combination is not a rhomboid, but an oblique 

 rhombic prism, since the planes make unequal angles at the three edges of the 



obtuse solid anjjles. 



