THE SIMPLE MICROSCOPE. 65 



value is intermediate between that of a double convex lens of 

 the best form and a doublet or achromatic lens. The doublet 

 m general use is that before alluded to as the invention of Dr. 

 Wollaston, and represented in section by fig. 40. It consists 

 of two planoconvex lenses, having their 

 focal lengths in the proportion of one to 

 three, or nearly so ; these are set in two 

 separate cells, a c, the upper one, a, is ca- 

 Pig. 40. pable of being moved up and down in c, 



by means of the screw, as represented by the figure ; this 

 enables the optician to adjust them to perform accurately. 

 The lenses are placed with their flat sides towards the object, 

 and the one of longest focus, which is also the largest, is 

 placed nearest the eye. Between the two lenses there is a 

 stop or diaphragm, b, which, for accurate definition, should 

 also be carefully adjusted. The doublet, as described by 

 WoUaston, in the Philosophical Transactions, was not pro- 

 vided with a stop, nor does he even allude to the introduction 

 of one; it is not certain, therefore, whether he was at all 

 aware of its value, and his bright career having terminated 

 in so short a time after the publication of the paper, the 

 omission may, in some measure, be accounted for. 



The form of doublet described, at page 29, as the invention 

 of Sir John Herschell, although free from aberration in the 

 centre of the field, has a great deal towards the margin, and 

 is therefore seldom used as a magnifier. 



When a triplet is required, it should be constructed on 

 the plan of that of Mr. Holland, first described in the forty- 

 ninth volume of the Transactions of the Society of Arts, and 

 before alluded to at page 31 ; it consists, as is shown in sec- 

 tion by fig. 41, of three planoconvex lenses, ab c, the first two, 

 a b, being placed close together, and the stop 

 or diaphragm between them and the third 

 lens, c; this combination of three lenses was 

 used by Mr. HoUand, either as a simple 

 microscope, or as an object-glass to a com- 

 Fig- 41. pound one ; and, although termed a triplet, 



it is essentially a doublet, having its front lens made up of 



