E. M. NELSON ON HISTORIC MICROSCOPY. 223 



The compound microscope was now abandoned in favour of the 

 simple lens, it being found that the image from a non-aplanatic 

 lens would not bear magnifying, the aberrations produced by the 

 lens being also magnified. 



Leeuwenhoek, in 1667, worked single lenses of high power, and 

 put a screw focussing adjustment to the instrument, which caused 

 the object to approach or recede from the lens. 



Sir Isaac Newton suggested reflecting microscopes in 1679, but 

 there is no acconnt of one having been made at that time. 



Stephen Gray used drops of water in 1696 ; his apparatus, like 

 that of Leeuwenhoek's, had a screw focussing arrangement. 



P. Bonnani, in 1698, produced a microscope with a coarse adjust- 

 ment and fine adjustment, a stage, and a condenser. It was very 

 rough, and the adjustments were hardly as fine as some of the pre- 

 ceding models ; but the principles are to be found in the micro- 

 scopes of the present day. 



J. Wilson introduced a simple microscope, with a screw focus, in 

 1702. These were of two forms ; one not unlike the seed micro- 

 scope now in use, and the other something like Leeuwenhoek's. 



These microscopes were very popular, but as they offer no new 

 link in the chain of development, we will pass over them without 

 any further description. 



We now come to the first compound microscope made for sale, 

 viz., that by J. Marshall, circa 1735. The stand was a wooden 

 box, with a drawer for apparatus in it. There was an upright 

 support fixed to this by a ball and socket joint. This support 

 carried the body and the stage. The stage could slide backwards 

 and forwards in a vertical direction. There were numbered divisions 

 marked on the upright support, which numbers corresponded with 

 numbers on the object glasses. By this means a coarse adjustment 

 was effected. If, for example, an objective was fixed to the body, 

 then the arm which carried the body was clamped to the upright 

 pillar at the marked division. The direct acting screw then made 

 the fine adjustment. There was a substage condenser formed of a 

 biconvex lens. 



The most important microscope of the time was, however, that 

 invented by Dr. Smith, of Cambridge, in 1738. He evidently saw 

 that the aberrations of the uncorrected object glass rendered com- 

 pound microscopes of little use. Leeuwenhoek's success with 

 single lenses, and the popularity of Wilson's adaptation of them, 



