E. M. NELSON ON HISTORIC MICROSCOPY. 227 



meniscus in 1821. Andrew Pritchard made a triplet, Blackie one 

 composed of a garnet, a quartz, and a flint glass, Sir D. Brewster 

 one of two fluids and a solid. 



Then comes the celebrated Wollaston doublet, in 1829, which 

 was like a small Huyghenian eye-piece, with the plane sides 

 of the lenses towards the object. This he fitted to a very neat 

 stand with a plano-convex condenser placed beneath the stage, so 

 that the object was in its focus. It had also a mirror, and rack- 

 work focussing. This instrument may be called the culminating 

 point of the simple microscope. We must now go back to the 

 compound microscope. Amici, who had been trying to achroma- 

 tize refractors, gave it up, and tried to improve reflectors. His 

 method was a modification of Newton's. Goring, Pritchard 

 and the Tulleys improved on it, and reflecting microscopes 

 reached their most perfect form in the Goring-Pritchard " engi- 

 scope," with metals made by Cuthbert. " Engiscope " is a new 

 word Dr. Goring tried to introduce for microscope ; it means 

 to see near things. Before leaving these antiques, let me mention 

 that in Pritchard's engiscope we first find Tyrrell's stage-move- 

 ments, which form is still retained by Messrs. Powell and Lealand. 

 The body of these instruments was fixed, and the stage focussed 

 instead. The stand was an ordinary telescope clip stand, with a 

 compass-joint, only it had a cruciform foot instead of the usual 

 tripod, a levelling-screw being placed in one of its legs. 



We now come to the last part of our subject, namely, the in- 

 vention of the achromatic objective for compound microscopes. It 

 was, as you may well conjecture, by no means the work of one 

 man, but the result of the combined action of many minds. I 

 shall not mention all the names which go to make up that great 

 list of workers to whom we are indebted for the beautiful instru- 

 ments we now have. As this paper is an abstract of types rather 

 than men, if I do not mention any notable worker in this field, it 

 is not because I undervalue his labours. 



In 1816, Frauenhofer, of Munich, made a single achromatic 

 lens, but its performance was considered inferior to the chromatic 

 lenses of that day. 



In 1823, M. Selligues, in France, made a lens of four achro- 

 matic combinations, which were for combined or separate use. It 

 does not appear that it was a very successful glass, but it was the 

 first to demonstrate the great advantage of combining achromatics 



