n8 SCIENTIFIC APPARATUS. 



line has not only the property in common with other doubly refract- 

 ing crystals, of polarising every ray which it transmits, but also it 

 absorbs the ordinary rays, and thus produces what is generally 

 required, a single beam of polarised light. Iceland spar, on the 

 other hand, generally produces two beams ; and various expedients 

 are adopted for getting rid of one of them : sometimes one of the 

 beams is shut off by a screen, but more often the ordinary beam 

 is thrown out of the field by means of the well-known instrument 

 called the Nicol's prism. In Norrenberg's instrument, as con- 

 structed by Hofmann, in Paris, as well as in another by Soleil, a 

 tourmaline is used as polariser, and a Nicol's prism as analyser. 



Duboscq has for many years past constructed a polariscope 

 for projection, by means of which every ordinary experiment in 

 polarisation may be thrown on the screen. The construction and 

 use of this instrument are alike rather difficult, owing to the small 

 size of the Nicols and double-image prisms employed ; but these 

 difficulties have been obviated by the construction of NicoFs 

 prisms of a larger size, viz. giving a clear field of 2 inches to 3^ 

 inches in diameter. A pair of such Nicols 2^ to z\ inch field, 

 with suitable lenses and other appliances, is exhibited; these were, 

 in fact, the first of this size, and their construction, under Mr. 

 Ladd, forms an epoch in the annals of polarisation. Another, 

 constructed by Ahrens, is exhibited; this, which has a field of 

 upwards of 3^ inches, is the largest and purest yet constructed. 



Besides the Nicol's prism there is also another by Foucault, in. 

 which the second ray is similarly thrown out of the field. This is 

 much shorter than the Nicol's prism, requires less spar, and is more 

 convenient for experiments on heat ; but in order to use advan- 

 tageously the entire field, the rays passing through it should be 

 strictly parallel. 



To Professor Jellet is due the invention of a new analysing 

 prism, by -means of which the plane of polarisation may be 

 determined with great precision. It consists of a long prism of 

 Iceland spar, which is reduced to the form of a right prism by 



