THE MICROSCOPE. 31 



or down by the pinion, K, whicli is tximed by the milled 

 head, L, The doublet has been before alluded to, it consists 

 of two planoconvex lenses, set each in a separate cell, M N. 

 The cell carrying the upper lens screws into that which 

 carries the lower lens, so that the distance between the indi- 

 vidual lenses may be regulated for perfect definition ; when 

 in use, the doublet is placed in a hole in the arm H. Since 

 Wollaston's time, the stand has been much improved, it has 

 been fitted up with an adjustable stage, and with fine and 

 coarse adjustments, and otherwise much altered in appearance; 

 but the one we have described is copied from his paper in the 

 Philosophical Transactions. A modification of this form of 

 instrument is at present in use, as an illuminator with many 

 microscopes, both simple and compound, and will be again 

 referred to in the chapter on "Illumination of Transparent 

 Objects." "With this microscope," Dr. WoUaston says "that 

 he was able to see distinctly the finest markings upon the 

 scales of the Lepisma and Podura, and upon those of the 

 gnat's wing." The doublet itself is, at the present time, much 

 employed, and is preferred by many to the compoimd micro- 

 scope for the examination of such objects as are perfectly flat, 

 and by reason of its portability, its value is much enhanced. 

 It is infinitely superior to a single lens, and is capable of 

 transmitting a pencil of an angle of 35° to 50° without any 

 sensible errors, and exhibits most of the test objects in a very 

 beautiful manner. 



The next great improvement in the single microscope, and 



the last we shall here notice, was effected by Mr. Holland in 



1832, and described by him in the forty-ninth volume of the 



Transactions of the Society of Arts. It consists, as shewn in 



fig. 20, of three planoconvex lenses, aic, the 



first two, a b, being placed close together, and 



the diaphragm or stop between them and the 



third lens, c. " The first bending," says Mr. 



Koss,* " being effected by two lenses instead 



J^ig- 20- q£ QQg^ jg accompanied by smaller aberrations, 



which are, therefore, more completely balanced or corrected at 



* Penny Cychpadia, Art., Microscope. 



