E. M. NELSOX OX EVOLUTIOX OF THE MICROSCOPE. 103 



concentric rotating stage, but also for an optical one — viz. the 

 placing of a diaphragm at the end of the tube or " canon." Let 

 any one try this plan with a non-achromatic lens and a blow-fly's 

 tongue, and they will soon find how much control can be 

 exercised over the mist and fog arising from the chromatic and 

 spherical aberrations of the uncorrected lens. 



Prof. Joblot also designed compound microscopes; these, 

 although their exteriors are ornate and of good artistic design, 

 are of a very crude type ; they possess, however, one point of 

 interest — viz. that there is stored in the covering cap of the eye- 

 piece a concave lens. This, when mounted in place of the convex 



Fig. 14. 



eye lenses, turned the instrument into what is now known as a 

 Briicke lens.* Prof. Joblot's book, which is profusely and well 

 illustrated, is divided into two parts ; in the first of these, which 

 deals mth objects seen with the microscope (principally those 

 obtained from decomposing infusions of various substances), 

 there is a water animalcule figured w^th a man's face on his 

 back ; all these plates were subsequently reproduced in Adams' 

 Micrographia Illustrata. The second part is entirely devoted 

 to microscopes, working drawings being given of most of them. 

 The date of, the first edition is 1718, and this is the date of 

 the microscopes. 



We now come to two catoptric microscopes, the first by Barker 

 in 1736 ; it is unnecessary to give a figure, for two reasons : first, 



* His figure is not very distinct, and his meaning with regard to this 

 subject is involved. 



