122 TRANSACTIONS OF THE 



made a twenty-sixth, magnifying nearly 1800 diameters, and tins 

 was generally considered, for some time, the utmost magnification 

 attainable. Not satisfied, Powell and Lealand have made since 

 then several object-glasses of only l-50th inch nominal focus, 

 magnifying with the lowest eye-piece 2500 diameters, and with the 

 highest eye-piece 20,000 diameters, or from 6J to 400,000 million 

 times, superficially. These glasses are indeed optical marvels; 

 they are excellent, but their use is limited to the smallest trans- 

 parent objects, as their actual working distance is only 0"003 of an 

 inch. Powell and Lealand have even made a l-80th inch object- 

 glass, but I am not able to speak of its optical performance. I 

 presume, however, that it is inferior to their celebrated l-50th, or 

 we would have heard more of it. 



It appears just now that the utmost, as regards both magnifica- 

 tion and definition, has been reached for the present, and until 

 the chemist will have supplied the optician with a denser trans- 

 parent medium of higher refractive power than any we can use 

 just now. Certain it is that microscopists and opticians will not 

 rest satisfied with the wonderful progress made lately, and that 

 the powers of microscopic definition and magnification will advance 

 more and more. The ne plus ultra of a microscope would be 

 an instrument with which we could see the ultimate particles of 

 matter, enabling us to verify, by ocular demonstration, our already 

 sufficiently proven theories of the co-relation of the great forces of 

 nature — heat, magnetism, electricity, and chemical afiinity — and 

 how they affect the molecules, whose shape would no longer 

 remain a hypothesis ; but so powerful a microscope will never be 

 constructed by human hands. 



I will now proceed to explain the construction of the compound 

 microscope, and of the various pieces of apparatus pertaining to it. 



The principal parts of a compound microscope are the object- 

 glass and the eye-piece, which are placed at the extreme ends of a 

 tube called the *' body." 



In order to explain the principle upon which the performance of 

 object-glass and eye-piece is based, I must interpolate in this place 

 a few remarks about light, which is not matter, but motion, and 

 consists of the undulations of a highly elastic fluid called tlie 

 luminiferous ether, which fills all space and matter in the uni- 

 verse. These undulations are extremely small, measuring from 

 the l-30,000th to the l-60,000th of an inch, according to the 



