176 Transactions of the Society. 



illumination with oblique rays of light, are amongst the most im- 

 portant. The alteration of the binocular system to suit a more or 

 less horizontal position of the tube and to correct reversal of the 

 image, has produced an instrument which gives the greatest 

 facilities to workers, and under which dissection and selection can 

 take place very readily. It supersedes the old plan of placing a 

 low-power objective on the draw-tube when using a low objective, 

 although this is an excellent plan. 



The improvements in the camera lucida are all in favour of the 

 low powers, and certainly during the past year this important 

 adjunct has been presented in many forms to the Society. One 

 apparently diminishes the almost inevitable distortion of the 

 reflected image, and another gives a great ifield with a very visible 

 end to the operator's pencil. 



The improvements in the manufacture of the polarizing ap- 

 paratus and the admirable substage movements, which have so 

 frequently been exhibited before the Society, raise the value of the 

 low powers in the important and most necessary study of rocks. 

 Finally, the ready change of objective by such methods as that of 

 Dr. Matthews, is gradually doing away with the nose-piece, an 

 adjunct to the Microscope which, if well made, is very useful, but 

 which if badly made is especially pernicious to the development of 

 the good performance of the low-power objective. 



Some important communications have been read before the 

 Society on the theory of the Microscope, and it is satisfactory to 

 acknowledge their great practical bearings. I allude especially 

 to the papers of Prof. Abbe and our Secretary, Mr. Frank Crisp. -■*. 

 Prof. Abbe's communications are full of interest. One relates 

 to the measurement of the refractive index and dispersive power of 

 fluids, without having resort to the old cumbrous methods by 

 hollow prisms. Abbe's refractometer is said to fulfil all that is 

 required of it. The leading principle of the apparatus depends 

 upon the obstruction of the rays by total reflection at the surface 

 of the fluid under examination. 



Wollaston and others adopted the method of observing the 

 maximum intensity of the reflected ray ; but a great advantage is 

 gained by observing instead, the maximum intensity, so to say, of 

 the transmitted ray. In the former case there is a difficulty in 

 ascertaining the precise point where the light reaches its maximum, 

 whilst in the latter a very small amount of light is easily detected 

 in the darkened field. A second communication is a continuation 

 of one on the Kational Balance of Aperture and Power, and relates 

 to the division of the entire power of the Microscope between 

 ocular and objective. The necessity for this balance was urged 

 in my last Presidential Address. 



Our Secretary's paper relates to the measurement of the magni- 



