Microscope Object-glasses and their Power. 69 



ticular to give the exact power of the ToUes' ^th, he remained 

 silent in regard to the Powell and Lealand -^^ih.. 



I applied one of the ordinary rules of arithmetic to Dr. Wood- 

 ward's figures, and therein, I consider, hes my unfairness. To 

 say that Messrs. Powell and Lealand did not know the power of 

 the objective when it was made, or that Dr. Woodward did not 

 know its power when he used it, would, I think, be unfair. 



However, I thank Mr. Wenham for coming to my rescue. He 

 says : " A scientific microscopist gives the diameters with his 

 illustrations, states the aperture, and the nominal power of the 

 object-glass. This quite meets the case." I agree with Mr. Wenham 

 perfectly. Dr. Woodward has given the diameters, and I have ap- 

 plied his own rule ; but in this case (the so-called -y^^) there was 

 a large discrepancy, amounting to nearly 50 per cent., between the 

 nominal and the actual power given. It was very easy to reach 

 the conclusion. I have placed the odium where I think it belongs. 



Mr. Wenham further says : " In such a difiicult and complex 

 arrangement as a high-power object-glass, it is almost impossible 

 for all the makers to work to the same magnifying standard." 

 I would like to inquire where all the much-talked-of mathematical 

 formulae are ? Does he mean to intimate that the best opticians 

 work by " rule of thumb " ? or that it is guesswork ? I have been 

 supposing all the time since I became interested in the use of the 

 microscope, and seeing such terms as "index of refraction," "dis- 

 persion," " light flint," " heavy flint," and " Faraday flint," " crown 

 glass," &c., that there imre some mathematical formulae involved 

 somewhere in the construction of object-glasses; possibly this 

 statement by Mr. Wenham may account for the discrepancy be- 

 tween the nominal and the actual power of some object-glasses, 



I should advise the opticians, if what Mr. Wenham says is 

 true, to wait until the object-glass is done, and then ascertain its 

 power and christen it, and then they would be within a reasonable 

 distance of the true standard. I am well aware that the position 

 of the combinations as determined by the cover adjustment for 

 difierent objects, will change the magnifying power to a certam 

 extent. I do not see why that range should exceed 15 per cent, 

 of the magnifying power of the objective (unless the object-glass 

 is made like the ToUes' |th for the U. S. Army Medical Museum, 

 to work both wet and dry with the same front). I have just tried 

 the following object-glasses, viz. Tolles' I, 70°; y%ths, 145^: 

 Ath, 170^ : Wales' ^th, 100° : Zentmayer, y\i\ 75° : all of these are 

 just on the mark in magnifying power, and in no case does the 

 power change over 12 per cent, of the actual power of the object- 

 glass, and the three lowest powers will adjust for an ordinary shde 

 bottom up. 



Now if we measure the power of an object-glass at its adjust- 



