THE OPTICAL EQUIPMENT 21 



pally due to the lower or higher degree of correction for 

 chromatic and spherical aberration. In the case of achromatic 

 lenses, the chromatic correction is only for two colours of the 

 spectrum, whereas spherical aberration is only corrected for one 

 colour ; so that there is always an outstanding portion of the 

 spectrum which the lens is not able to deal with properly, and 

 which, therefore, is not utilised in the formation of the image. It 

 becomes evident either as a more or less visible fringe of colour 

 around the image of the object, or is diffused over the whole as a 

 general haze with resulting loss of quality of the image. 



Apochromatic lenses are chromatically corrected for three 

 different colours of the spectrum, and for spherical aberration 

 are corrected for two colours. For visual work, and more parti- 

 cularly for photographic purposes, this is, of course, a very great 

 advantage ; but, in addition, the higher numerical aperture 

 which owing to their construction it is possible to attain in 

 them, gives them a proportionately greater resolving power. A 

 greater amount of light is transmitted by the objective, and 

 consequently in photography the exposures are shortened and 

 the resulting image is of greater sharpness and purity. Multi- 

 coloured objects assuming of course that all portions of them 

 are in the same object-plane are all equally sharp and are 

 focussed in the same optical plane. The consequence is that 

 the image differs little in quality whether white or monochro- 

 matic light is used, and differences in colour do not entail any 

 alteration in the focus of the objective. 



A point of first importance in choosing an objective is to 

 ascertain its numerical aperture ordinarily referred to as its 

 N. A. Until comparatively recently a quality known as penetra- 

 tion, or what is usually referred to in photographic lenses as 

 depth of focus, was regarded as a particular advantage, and in 

 fact was sometimes referred to as an especially good point in a 

 lens. This is no longer the case at least for any work of even 

 an approximately critical nature since the penetration of a 

 microscope objective, so far as it exists, decreases as numerical 

 aperture is increased. In any case penetration is a delusive 

 quality, and it often appeared to exist when actually perfect 

 definition was not obtainable in any plane ; but as there was 

 no absolute sharpness of the image for comparison the eye was 

 deceived, and might even be said to imagine that such equal 



