75G 



in this way, not only exhibits a double set of lines, but also transverse 

 lines, giving" the whole the appearance of being covered wtth a deli- 

 cate net-work. Four drawings of this object were exhibited, showing 

 it in as many different positions, making a complete revolution of the 

 field in which the the markings first mentioned were distinctly visible. 

 In order to bring out these appearances it is necessary that the light 

 should be very oblique, and must be passed laterally through the 

 "bull's eye" in sucli a manner that the object (the Navicula) may 

 appear of an intensely blue colour, nearly opaque. The stage is then 

 to be gradually turned round until the shell is in the position to be 

 best seen as described. 



December 9tJt, 184(). — J. S. Bowerbank, Esq., F.R.S., President, 

 in the chair. 



A paper " On the application of Polarized Light in Microscopic 

 ObservatiouLi," by Mr. Legg, was read. After noticing the remarks 

 of Dr. Brewster respecting the advantages likely to be derived from 

 the application of polarized light in the microscopic examination of 

 delicate structures, Mr. Legg described a series of pojarizing appa- 

 ratus which may be readily adapted to almost any microscope, con- 

 sisting, 1st, of a bundle of plates of crown glass, from which the 

 light is to be reflected at an angle of 56°, in which position one por- 

 tion only of the light is refracted and another transmitted, each of 

 which portions consists of light polarized in opposite planes. This 

 arrangement 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, 

 inasmuch 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 em- 

 ployed 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 —^ to 5- of an inch in diameter. In 

 the first of these experiments, in which the doubly refracting crystal 



