PROCEEDINGS OF THE SOCIETY. 271 



the fingers ; and in putting the instrument away, a handle which did 

 not interfere with the fine-adjustment was a great advantage. He was 

 very glad to find that the makers were beginning to introduce this. He 

 thought, also, that having the fine-adjustment made so that it could be 

 worked from both sides would often be found a great convenience, and 

 Mr. Curties was to be congratulated upon having introduced this new 

 model. 



Mr. Rousselet said that the addition of this convenient handle to a 

 Microscope was not a particularly recent invention, for John Cuff made a 

 handy Microscope with a handle about 1750. 



Dr. J. W Evans, LL.B., F.G.S., gave a lecture on "The Optical 

 Examination of a Crystal Section in a Rock Slice," which was illustrated 

 by a large number of lantern slides and also by a projection polariscope. 

 devised by Mr. Cheshire. Having shown and explained the construction 

 .and use of polarising Microscopes — both those with the analyser in the 

 usual position above the objective, and those with it placed above the 

 eye-piece — he described a systematic method of determining the different 

 crystallographic and optical directions in the section by bringing them 

 in turn parallel to the right and left cross wire, the direction of vibration 

 in the lower nicol. Each direction is denoted by the index-reading 

 when it is in this position, and all the particulars are embodied in a 

 sketch representing the position of the section when the index-reading 

 is zero. The interference colours due to the relative retardation of the 

 light vibrating in the two directions of " extinction " were shown in the 

 case of crystal plates, mica steps, and quartz wedges. On the addition 

 of a prism the spectra of these colours were obtained, showing them 

 to be due to the presence of dark bands which moved from the violet 

 to the red end in ever-increasing numbers, as the thickness of the 

 crystal increased. The method of calculating the thickness of a crystal 

 plate from the interference colours shown by a known mineral was ex- 

 plained, as was also that of ascertaining the birefringence of an unknown 

 mineral from the colours it exhibits, and the thickness of the rock-slice. 

 The application of the Becke method of determining the relation of 

 the refractive index of adjoining minerals was also explained, as well as 

 the use of immersion liquids in order to obtain the exact refractive index 

 of minerals on the margin of a slice by determining that of the liquid 

 which has the same refractive index, by means of the Abbe or Herbert 

 Smith refractometer. 



The Chairman having been obliged to leave the Meeting before the 

 conclusion of the lecture, the Chair was at his request afterwards 

 occupied by Mr. A. N. Disney, who moved a hearty vote of thanks to 

 Dr. Evans for his lecture. This, on being put to the Meeting, was carried 

 unanimously. 



Mr. C. F. Rousselet read a paper " On Synchaetafennica sp. n., and on 

 the Resting-egg of S. pectinatci" Specimens of S.fennica and S. bicornis, 

 also a drawing with which the paper would be illustrated, were exhibited 

 in the room. 



The thanks of the Society were voted to Mr. Rousselet for his paper. 



