24 



CONSTEtJCTION OP THE MICEOSCOPE. 



may be formed out of a combination of lenses, as will be hereaft'ep. 



We have hitherto considered a lens only in reference to its enlarge^ 

 ment of the object, or the increase of the angle under which the object 

 is seen. A further and equally important consideration is that of the 

 number of rays or quantity of light by which every point of the object 

 is rendered visible ; but it at the same time becomes as important that 

 we do not sacrifice definition to this end. Much may be accomplished^ 

 as we have before pointed out, by the combination of two or more 

 lenses instead of one, thus reducing the angles of incidence and re- 

 fraction. 



The first satisfactory arrangement for this purpose was the inven- 

 tion of the celebrated Dr. WoUaston. His doublet (fig. 20) consisted of 



two plano-convex lenses 

 having their focal lengths 

 in the proportion of one 

 to three, or nearly so, and 

 placed at a distance which 

 can be ascertained best by 

 actual experiment. Their 

 plane sides are placed 

 towards the object, and 

 the lens of shortest focal 

 length next the object. 



It appears that Dr. 

 Wollaston was led to this 

 invention by considering 

 that the achromatic Huy- 

 ghenean eye-piece, which 

 will be presently de- 

 scribed, would, if revers- 

 ed, possess similar good 

 properties as a simple mi- 

 croscope. But it will be 

 evident, when the eye- 

 piece is understood, that 

 the circumstances which 

 render it achromatic are 

 very imperfectly applic- 

 able to the simple microscope, and that the doublet, without a nice 

 adjustment of the stop, would be valueless. Dr. Wollaston makes no 



fig. 20. 



