M. A. AINSLIE OX AX ADDITIOX TO THE OBJECTIVE. 565 



that it is the best that the objective will do. A perfect image 

 is only formed, for any given thickness of the cover-glass, at one 

 particular distance from the back lens, and at no other. 



To take a numerical example, which will probably help to make 

 this point clearer, suppose we have a cover-glass so thick that 

 the correct tube-length is no more than 100 mm. It is obvious 

 that the tube cannot be closed down as much as this, except on 

 a stand of very exceptional construction. 



To treat the objective for the moment as an animate thing, 

 we can as it were leave it under the impression that it is forming 

 the image at the point where it can do so best, i.e. at 100 mm. 

 from the bottom of the tube. If we now introduce behind the 

 back lens a concave lens of low power, we decrease the conver- 

 gence of the beam projected by the objective. This in no way 

 affects the working of the objective, since the action of this new 

 lens does not commence until the objective has finished its work ; 

 but if the power of the additional lens is suitably chosen, we 

 can so alter the degree of convergence of the beam of light as 

 to make it come to a focus, say, at a distance of 170 mm. from 

 the back lens that is to say, well within the limits of an ordinary 

 draw-tube. We have, in fact, altered the tube-length for which 

 the objective is corrected. If the thickness of the cover-glass 

 be still further increased, we have only to introduce a lens of 

 shorter focus, and therefore of greater power, to bring the image 

 within the limits of the draw-tube as before ; and it will readily 

 be seen that this gives us the power, within somewhat wide 

 limits, of obtaining good definition through a thickness of cover- 

 glass which would in ordinary conditions be a complete bar to 

 anything like good definition. 



Conversely, if the cover-glass is inordinately thin, the distance 

 at which the perfect image will be formed may be considerably 

 beyond any length that the draw-tube will reach ; but the 

 introduction of a convex lens of suitable power will increase the 

 convergence of the beam of light, and so bring the image to a 

 distance at which the draw-tube can deal with it. 



Figs. I. and II. are intended to illustrate the action of the 

 additional lens ; in each figure, 

 P is the object ; 



O is the objective, shown diagrammatically as a single lens 

 C is the cover-glass (thick in I., thin in II.) ; 



