CORRECTION OF LENSES 49 



ground glass ; the images formed by other rays would 

 not be in accurate focus at the same time, but they are 

 less luminous and may not detract very much from the 

 sharpness of the composite image on the ground-glass. 

 On the other hand, the effect produced by the different 

 coloured images on a photographic plate is, under 

 ordinary circumstances, quite different from that produced 

 on the eye. The blue rays are the most actinic, and 

 consequently the indistinct image they form, though 

 not noticed when focussing, is registered more easily by 

 the plate than the sharp yellow-green one, and gives a 

 fuzzy photograph. 



The Correction of Lenses. There is a wide selection of 

 glasses suitable for lens-making, each with its own 

 special physical properties Refractive Index, Dispersive 

 Power, etc. and a lens in which the errors are more or 

 less eliminated can be constructed by a proper combina- 

 tion of two or more simple lenses whose shapes and 

 materials have been carefully calculated and selected so 

 that their errors balance. 



The Refractive Index of any medium is the sine of the 

 angle made by a ray of light in vacuo (or practically in 

 air) incident upon its surface, and the perpendicular to 

 that surface, divided by the sine of the angle made by 

 the same ray to the perpendicular, after it has passed into 

 that medium. It will make the methods of correction 

 clearer if figures are given for a typical crown and flint 

 glass : 



Ref. Index of Crown Ref. Index of Flint 

 Glass. Glass. 



For a red ray " C " 

 yellow " D " 

 blue-green " F " 



1-515 

 1-518 

 1-524 



1-710 

 1-717 

 1-735 



The Dispersive Power measures the difference between 

 the refractions of two selected rays compared with that of 



