MICROSCOPES. 199 



I will now proceed to say a word or two with regard to the 

 construction of the instrument itself, and the general mechanical 

 arrangements requisite to hold the optical part and the object 

 under examination, and for illuminating it properly. The 

 Exhibition contains as simple a compound microscope as is 

 possible, made by Janssen, the first ever constructed. There is 

 no stage and no mirror to reflect the light, merely a simple 

 lens for 'object-glass and a single lens for eye-piece, held 

 in tin tubes. There are also many kinds of most inter- 

 esting microscopes, of nearly all periods down to the most 

 modern, but I will allude to only a few to illustrate some 

 particular points. Passing down from the one just named 

 by Janssen, we come to those forms of instrument which are 

 especially characterized by very large eye-pieces of two lenses 

 on Huyghens's principle. As an example, I may refer to the 

 two microscopes made by Marshall in the commencement of the 

 eighteenth century, the eye-pieces having field-lenses of two 

 inches in diameter, which makes the instrument look most 

 remarkably stout. Few microscopes have been made larger 

 or more complex than those by Martin in the middle of the 

 eighteenth century. One of these in the Exhibition belongs 

 to the Royal Microscopical Society, and it was thought to be 

 the only one in existence, but another has turned up which 

 I had the opportunity of putting together ; I must confess 

 I had very great difficulty in finding out where the different 

 things fitted. There is almost every movement that any one 

 could possibly devise, and a great many of them for the 

 practical work of the present day are not only useless but far 

 worse than useless. Then again the construction is exceed- 

 ingly curious. One tube goes into another for only about one- 

 eighth of an inch ; and there is scarcely a firm joint in the 

 whole apparatus. There are movements of nearly every 

 decription ; you can move the body of the instrument up 

 and down, from side to side, and backwards and forwards, 

 but there is scarcely a single joint which one could call 

 good. These instruments may be looked upon as the 

 opposite extreme of that by Janssen. 



Leaving the microscopes of mere historical interest and 

 coming down to modern times we have examples of the 

 most simple and complex arrangements. Some are of the 

 very simplest form, more in vogue on the Continent than in 

 this country ; so constructed that you have not the power of 



