ZOOLOGY AND BOTANY, MICROSCOPY, ETC. 129 



in Mr. S. Wells's micro-pliotograph exhibited at the Society in 

 1876* 



If, among the above results, we select as a standard Dr. Woodward's 

 micro-photographs of A. pellucida, produced with Zeiss's yV oil- 

 immersion, we shall find that (apart from the use of an amplifier) the 

 magnification was obtained by receiving the image at a conjugate 

 distance equivalent to the use of an eye-piece magnifying 25 linear — 

 that is to say, the yV would give 120, and this combined with an eye- 

 piece of 25 would produce 3000. An eye-piece of 25 being of f 

 equivalent focus, it follows that the eye-piece {-^-^) referred to by the 

 lady correspondent is just 20 times as strong (500 as against 25) as 

 that which Dr. Woodward thought most effective to exhibit the best 

 resolving power of Zeiss's yV. 



Applying similar reasoning to Mr. Charles Stodder's example, i. e. 

 ToUes's i giving 550 linear, we learn that he obtained his " best result 

 on record " with a.r> eye-piece of about 9 linear, that is, less than 

 1 inch equivalent focus — a startling difference when compared with 

 a 3^0 with its 500 linear. 



Mr. Dallinger referred to 10,000 or 15,000 diameters in his lecture 

 at the Eoyal Institution and at Cambridge (1879), but when describing 

 his results in his paper to this Society,| he pointedly stated that his 

 hest results were obtained with Powell and Lealand's new formula 

 •| immersion, and his magnifications were less than 4000 — that is to 

 say, his eye-piece power did not exceed 50 linear. 



Here, then, we have roughly collected a few instances of practical 

 results, to which we draw special attention, as clearly indicating 

 that in the opinion of some of the best living manipulators with the 

 Microscope, amplifications beyond 5000 or 6000 linear exhibit no 

 further visible resolving power ; indeed we think these figures are 

 far in excess of the practically useful limit, and that from 2000 to 

 3000 linear would amply represent the limit of visible resolving 

 power. 



In America, more than one professed microscopical expert — notably 

 Mr. John Phin, editor of the ' American Journal of Microscopy/J and 

 " Carl Eeddots," §— has lately alluded to 80,000 or 100,000 diameters 

 as within the power of his appliances. With the evidence of micro- 

 photographs before us, we must regard any such magnification as of 

 no practical scientific value. We say this the more advisedly from 

 the fact that we have tested some of the finest specimens of American 

 optical work, including the choicest objectives from the hands of 

 Tolles, Spencer, and Wales ; we have also tested the best lenses we 

 have met with of Powell and Lealand, Zeiss, Hartnack, Prazmowski, 

 Nachet, Seibert and Krafft, Beneche and others, and our conclusion is, 

 that the use of any such eye-pieces as -^-^ cannot be regarded as any- 

 thing more than microscopical eccentricity, originating probably from 

 that very common popular error of making the value of a Microscope 

 to depend exclusively upon its magnifying power. 



* Mon. Micr. Journ., xvi. (1876) p. 1G9. t Ibid., xiv. (1875) pp. 105-8. 



X Eng. Mech., xxxi. (1880) p. 469. § Amer. Mon. Micr. Journ., i. (1880) p. 33. 

 Ser. 2.— Vol. I. K 



