362 -PHYSIOLOGICAL PHYSICS. t cha p- xxvm. 



spherical, but also from chromatic aberration, 

 Huyghens himself, it appears, was unaware that his 

 eye-piece served both purposes. It was applied to 

 the microscope by Campani. 



Hiiygliens' eye-piece consists of two plano- 

 convex lenses fitted into one tube at some distance 

 from one another. The plane surface of each lens is 

 towards the observer's eye. The distance between 

 the two should be equal to half the sum of their focal 

 length. The disposition of the two lenses is such that 

 the aberration of one corrects that of the other. The 

 first lens disperses the rays from the object, but the 

 dispersed rays by passing through the eye lens are 

 rendered parallel. They appear to the eye on that 

 account to come from the same point ; the different 

 colours, therefore, coincide, and a white, instead of a 

 coloured image, is the result. Between the two 

 lenses there is a stop, which cuts off outside rays, and 

 so the aberration' of sphericity, as well as that of 

 chromatism. is got rid of. 



Immersion lenses. The more one increases 

 the magnifying power of a lens the shorter becomes 

 the focal distance. The more nearly the object ap- 

 proaches to the objective, the more obliquely do the 

 rays proceeding from it fall upon the object-glass, the 

 fewer rays are able to pass through the system of 

 lenses, and the weaker is the illumination. Besides, 

 the shorter the focal length becomes the greater is the 

 difficulty of obtaining cover-glasses of sufficient thin- 

 ness to interpose between the object and the object- 

 glass. Amici conceived the idea of placing on the 

 cover-glass a drop of water or other liquid into which 

 the first lens of the object-glass dips. The rays of 

 light passing from the object through the cover-glass 

 into the water are less refracted than if they passed 

 through the cover-glass into air. In the former case 

 the rays fall less obliquely on the object-glass, and are 



