84 THE mCEOSGOPE 



It is a well-known fact that glass refracts the various colours 

 by a different amount, and consequently a single lens wiU not give 

 an image free from colour because it has different foci for different 

 colours, the focus for red in a positive lens being further from tiro- 

 lens than the focus for blue. This property of refracting colours 

 by a difierent amount is caUed the dispersion of the glass. From 

 the time of Newton to that of Dolland it was supposed that the 

 dispersion of difierent glasses was proportionate to their refrac- 

 tive powers (/x-1), and therefore proportionate to their foci. 

 In other words, it was thought that any positive and negative 

 lenses which had the same efiect on the colour must have the 

 same effect on the focus whatever glass they were made from, 

 and that combining two such lenses would give the efiect of a 

 plain piece of glass without any focus. Conversely, they supposed 

 that any combination of lenses which had a focus must have a 

 colour aberration equal to a single lens of that focus. 



In the middle of the eighteenth century Dolland discovered 

 that this was not so, and that whilst the flint then is use had as 

 compared with the crown glass a refractive power of 60 to 50, 

 it had a dispersive power of 60 to 36. 



If we take a crown glass of 1-5 refractive index, the difierence 

 in focus between the pale yellow or C-rays, and the green or F- 

 rays, is about 1/60 of the focus ; but if we take a flint glass of about 

 1-6 refractive index, the difierence between the corresponding 

 rays is about 1/36 of the focus. Therefore, if we take a positive 

 lens of crown glass which is 36 inches focus, and a negative 

 lens of flint glass which is 60 inches focus, the colour will 

 be corrected for these two rays when the two lenses are put 

 together, and the result wiU be an achromatic lens of 90 inches 

 focus. 



Although in this combination the two rays C and F would 

 be correct, it would not give a perfect correction for the other 

 parts of the spectrum, because the refractive power of the two 

 glasses is not quite regular for the difierent colours. 



For instance, the four coloiu:ed rays known by the 

 spectrum lines C, D, F, and G do not have proportional disper- 

 sions ; if we call the dispersion from C to F 1,000, we find that in 

 a hard crown the distance from C to D is 295, and from F to G 

 568, whilst in a medium flint the distances are respectively 285 

 and 608. This may be expressed in the following manner. 



Hard crown . 

 Medium flint . 



