CONSTRUCTION OF THE MICROSCOPE. 31 



instrument. The simple microscope may consist of one, 

 as seen in fig. 1 9, or of two or three lenses ; but these 

 latter are so arranged as to have the effect only of a single 

 lens. In the compound microscope, not less than two 

 lenses must be employed : one to form an inverted image 

 of the object, which, being the nearest to the object, is 

 called the object-glass; and the other to magnify this 

 image, and from being next the eye of the observer, called 

 the eye-glass. Both these may be formed out of a com- 

 bination of lenses, as will be hereafter seen. 



We have hitherto considered a lens only in reference to 

 its enlargement 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 ; and 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 refraction. The first satisfactory arrange- 

 ment for this purpose was the invention of the celebrated 

 Dr. Wollastou. His doublet (fig. 20) consisted of two 

 plano-convex lenses having their focal lengths in the pro- 

 portion of one to three, or nearly so, and placed at a 

 distance which can be ascertained best by actual expe- 

 riment. 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 Huyghenean eye- 

 piece, which will be presently described, would, if reversed, 

 possess similar good properties as a simple microscope. 

 But it will be evident, when the eye-piece is understood, 

 that the circumstances which render it achromatic are 

 very imperfectly applicable to the simple microscope, and 

 that the doublet, without a nice adjustment of the stop, 

 vrould be valueless. Dr. Wollaston makes no allusion to 

 a stop, nor is it certain that he contemplated its intro- 

 duction ; although his illness, which terminated fatally 

 soon after the presentation of his paper to the Eoyal 

 Society, may account for the omission. 



The nature of the corrections which take place in the 

 doublet is explained in the annexed diagram, where lo I' is 



