368 OBJECTIVES, EYE-PIECES, THE APERTOMETER 



lens, which might be a triplet, will require to have +'214 V 



of spherical aberration to render the whole combination aplanatic. 



( hi the whole, and for the purposes of practical and prolonged 

 biological investigation, it is to the dry apochromatics that we are 

 most indebted, and from their use we shall derive the largest benefit. 



As no subject is really of more importance than a clear under- 

 standing of the difference of action of chromatic, achromatic, and 

 apochromatic lenses, we venture to present a diagrammatic illustra- 

 tion, which, while not strictly accurate, will carry with it no error, 

 MS ;i popiilar illustration of this important subject. 



In fi ". 321, 1, 2, 3, we have representations, as truly as they can 

 be drawn, of zones of equal light ; that is to say, the peripheral zone 

 will transmit an amount of light equal to that given either by the 

 intrrmt'(li;ite zone or the central circle. Let them therefore be 

 called equilucent zones. 



/. 2. 3. 



FIG. 321. 



If we assign a numerical value for the visual intensity of the 

 whole spectrum, say 100, made up of the following parts, viz. : 



Red 



Orange-yellow 

 Yellow-green . 

 Blue 



15 

 40 

 30 

 15 



then if in any one of the equilucent zones the whole spectrum is 

 brought to a focus, we shall have for that zone 100 as its effective 

 value. 



But the entire object-glass is divided, as in the diagram, into 

 three eipiilucent /.ones ; consequently 300 will represent the value of 

 the whole lens, provided the whole of the spectrum is brought to the 

 saint- focus. 



l!y referring to the diagrams we see that in anon-achromatic 

 lens (fig. 321, 3) we shall get only 40, because only one part of the 

 spectrum is brought to the focus in its intermediate zone; and as 

 -pliei-ica! aberration causes the light which passes through the other 

 zones to lie brought to other foci, they for all practical purposes migl it- 

 be Stop | ied out. 



In the achromatic- lens we have (lig. 321, 1) in the intermediate 

 /.one t\vo parts of the .spectrum combined, as 40 + 30=70, and one 



