134 THE HISTORY AND DEVELOPMENT OF THE MICROSCOPE 



This system of focussing was employed in a more practical form 

 by Hartsoeker in 1694 ami was adopted by Wilson in 1702. It 

 became a very popular form for the microscope in the eighteenth 

 century. 



We are indebted to Bonamms also for originating a horizontal 

 form of microscope, which is interesting and which, in a drawing of 

 the instrument, is shown to possess a sub-stage compound condenser 

 fitted with focussing arrangements for illuminating transparent 

 objects. There was great convenience in using the microscope in a 

 horizontal position with a lamp and condenser in the same axi>. 

 especially as all the compound microscopes previously constructed 

 had been employed vertically, or had been directed towards the sky 

 for purposes of illumination. Remarkably crude as the mechanism 

 appears, it is a very early instance of the use of what has become 

 though slowly and late on the continent a now universally acknow- 



FIG. 102. Hartsoeker's simple microscope (1694). 



ledged optical arrangement indispensable for the best results, viz. a 

 compound condenser fitted with focussing mechanism for illuminating 

 transparent objects. The picture of the entire instrument is shown 

 in fig. 101. 



In Hartsoeker's microscope 'the lens-carrier A B, fig. 102 (on 

 which the cell P, containing the lens, is screwed), screws into the 

 body O C, Q Dat Q ; the thin brass plates E and F fit within the 

 body, the portions cut out allowing them to slide on the short pillars 

 O C and Q D, and the spiral spring pressing them towards C D ; 

 the object-slides, or an animalcule cage G H (hinged at a b to allow 

 the lid G to fit into H, enclosing the objects between strips of talc), 

 slide between the plates E and F when in position, and the " screw- 

 barrel "IK fits into the screw-socket C D and regulates the focus- 

 sing ; a condensing lens, N, fits, on a second " screw-barrel," L M, 

 which is applied in the screw-socket of I K. This arrangement of 



