1620 Microscopical Society. 



ment is the best adapted to low single powers. 2ndly, a plate of 

 tourmaline, as free from colour as possible, and cut parallel to the 

 crystalline axis ; and 3rdly, a Nicol's or single image prism, being a 

 portion of a crystal of Iceland Spar, cut, and combined with a piece 

 of glass, so as to throw out of the field of view one of the two images 

 produced by the double refraction of the crystal. This he described 

 as being the most eligible for the compound microscope, in as much as 

 it is perfectly free from colour, and requires very little adjustment. 

 He then described a series of experiments, illustrating the most 

 striking phenomena of double refraction, in which he employed the 

 Nicol's prism adapted under the stage. A double refractor, adapted 

 to the eye-pieces, a film of selenite of uniform thickness, placed in 

 accordance with its crystalline axis, and a plate of brass, perforated 

 with holes from about one-sixteenth to a quarter of an inch in diame- 

 ter. In the first of these experiments, in which the double refracting 

 crystal was placed over the eye-piece, two distinct images appeared, 

 one of which revolved round the other when the eye-piece was turned 

 round, thus showing the ordinary and extraordinary rays. On the 

 second, the Nicol's prism was applied under the stage, the other ar- 

 rangements remaining the same. Upon turning the eye-piece, al- 

 though two images are produced, but one is seen when half the revo- 

 lution is performed, i. e. at 180° from the first position. Changes 

 also take place at every other quadrant. In the third experiment 

 the selenite plate was interposed, the images were now coloured and 

 presented the complimentary colours at every quarter of a circle. 

 When the hole in the piece of brass was of a large size, the images 

 were seen to overlap and white light was produced. The author con- 

 cluded with some remarks upon the service likely to be rendered to 

 microscopical investigation by the employment of polarized light. 



============ J ' W 



Further notes on the Bones of Extinct Deer found in Ireland. 



[The following letters are addressed to the Editor of the Farmer's Gazette, and 

 appear in the columns of that highly useful periodical.] 



1. From Professor Owen of London. 



London, Dec. 7th, 1846. 

 Sir, — As you have done me the honor to refer to my work on British 

 Fossil Mammalia [Zool. 1595], in regard to the question of the anti- 

 quity of the magnificent extinct species of deer [Megaceros Hiberni- 

 ans) — the abundant remains of which are so characteristic of Ireland 



