THE MICKOSCOPE. 11 



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 Huyghenean Eye-piece, which 



will be hereafter 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, 



^-r would be valueless. Dr. Wollaston makes no 



Fig. 4. allusion to a stop, nor is it certain that he contem- 

 plated its introduction, although his illness, which 

 terminated fatally soon after the presentation of his paper, 

 may account for the omission. 



The nature of the corrections which take place in the 

 doublet is explained in the annexed diagram (Fig. 5), where 

 L O L' is the object, P a portion of the pupil, and D D the 

 stop, or limiting aperture. 



Now, it will be observed that each of the pencils of light 

 from the extremities L L' of the object is rendered eccentrical 

 by the stop, and of consequence each passes through the two 

 lenses on opposite sides of their common axis O P; thus each 

 becomes affected by opposite errors, which to some extent 

 balance and correct each other. To take the pencil L, for 

 instance, which enters the eye at R B, B, B ; it is bent to the 

 right at the first lens, and to the left at the second; and as each 

 bending alters the direction of the blue rays more than the 

 red, and, moreover, as the blue rays fall nearer the margin of 

 the second lens, where the refraction, being more powerful than 

 near the centre, compensates in some degree for the greater 

 focal length of the second lens, the blue and red rays will 

 emerge very nearly parallel, and of consequence colorless to 

 the eye. At the same time the spherical aberration has been 

 diminished by the circumstance that the side of the pencil 

 which passes one lens nearest the axis passes the other nearest 

 the margin. 



This explanation applies only to the pencils near the extrem- 



