324 Address by the President. [Sess. 



succeeded in neutralising to some extent the spherical aberra- 

 tion. 



Chromatic Aberration. — This arises from the composition of 

 the light. White light, as we understand it, is made up of 

 what are called the seven primary colours — red, orange, 

 yellow, green, blue, indigo, and violet. Of these, red is the 

 least refrangible and violet the most refrangible, the others 

 being intermediate in the order stated. When this light is 

 passed through a convex lens, the violet rays come to a point 

 nearest, and the red to a point farthest from, the lens. The 

 diagram on Plate XXV., Fig. 2, will make this clear. 



The white light A A" falling upon the peripheral portions of 

 the lens B B" is broken up, so that the violet rays come to a 

 focus at C, and then pass on to F F, while the red rays only 

 come to a focus at the point D, much farther from the lens ; 

 the other rays coming to foci at points intermediate between 

 C and p. This difference of foci between the various coloured 

 rays is termed chromatic aberration, which causes the object 

 when viewed through the lens to be blurred by different 

 colours of the spectrum. The great endeavour of the prac- 

 tical optician was to get rid of this colouring, and thus show 

 the object in clear outline. This was accomplished to a very 

 great extent by a combination of lenses, some convex and 

 others concave, and some made of crown-glass and others of 

 flint-glass. By this means the opticians were able to com- 

 bine two rays of the spectrum coming to foci between the 

 violet and the red. In the construction of a microscopic 

 object-glass, as many as six lenses were used, usually combined 

 in pairs. One maker employed seven ; this was by making 

 the front a triple combination, which consisted of two plano- 

 convex lenses of crown-glass with a plano-concave of flint-glass 

 between them. The microscopic object-glasses of this order 

 are called achromatic — that is, without colour. Names which 

 stand out in bold relief in those early days of achromatic 

 object-glasses are Lister, Eoss, Powell, and Smith & Beck. 

 The dates were between 1830 and 1840, and it is not too 

 much to say that the work of these opticians with the material 

 at their command has never been surpassed. 



To preserve microscopic preparations permanently, they are 

 covered with a piece of very thin glass, which is called the 



