THE PARTS OF A LANTERN 21 



model for a lantern condenser. Some opticians including 

 Mr. Dallmeyer prefer to make the lens next the slide a 

 ' crossed ' lens ; but practically there is no advantage in this, 

 and it (slightly) diminishes the size of the slide which a given 

 condenser will cover, as already indicated. 



There remains to consider the chromatic aberration ; for 

 of course the margin of a lens acts precisely like a prism, 

 and makes the different colours diverge into a spectrum, as is 

 easily seen by experiment. Now these diverging rays from 

 the first lens, falling on the second lens, are by it more or less 

 converged, like any other rays diverging at the same angle. 

 The amount of this convergence is greater, the farther the 

 rays are allowed to proceed before convergence by the second 

 lens ; the conjugate foci are altered in relation to each other, as 

 in other cases. Hence there is one particular distance at which 

 the differently- coloured rays dispersed by the first lens are 

 made approximately parallel by the second lens for a given posi- 

 tion of the radiant ; and at this distance between the lenses, 

 the condenser becomes very nearly achromatic, only the nar- 

 rowest line of colour being visible at the extreme edge of the 

 illuminated disc. The exact posi- 

 tion is a matter of experiment or 

 calculation for a given glass ; but 

 practically, in a condenser of the 

 kind here described, the clear dis- 

 tance between the lenses is usually 

 between a quarter and three- 

 eighths of an inch for a four-inch 

 condenser. Such a condenser is 

 shown in fig. 13, and will work " pm 13 



exceedingly well through a wide 



range of foci. For constant long-focus work say with objec- 

 tives over ten inches focus the second lens may be a ' crossed' 

 lens, with the deepest curve insid.e, but for all-round work, two 

 planes will be found best, as the slide may be brought close 



