ZOOLOGY AND BOTANY, MICKOSCOPY, ETC. 697 



analogy of the phenomena presented by large objects, and with the 

 assumption that the same phenomena must hold good in the other 

 case also.* 



Curiosities of Microscopical Literature. — One would hardly have 

 expected to find such a paragraph as the following in a book pub- 

 lished in London in 1881, even although written " without assum- 

 ing the possession on the part of the reader of other attainments than 

 those possessed by the average schoolboy or schoolgirl " : — " In the 

 " same year (1824) Tulley, of London, succeeded in constructing for 

 " the first time in England an object-glass of 3 lenses. Sir John 

 " Herschel, Professor Airy, and Professor Barlow [no mention of 

 " Lister !] furnished valuable contributions to the theory of the 

 " achromatic object-glass. More recently a suggestion of Sir David 

 " Brewster's has been carried out, by the construction of lenses of 

 " diamond. By these and other modern improvements, especially in 

 " the mode of illuminating the objects, investigations are now carried 

 " into structures so minute that magnifying powers of 2000 or 3000 

 " diameters have to be used "If 



The suggestion of diamond lenses was abandoned more than fifty 

 years ago, j and none of the present generation of Microscopists have 

 ever had an opportunity of testing tbe " improvement " which it is 

 suggested the diamond has been to microscopical investigation. 



When will popular writers get to understand that neither the 

 size nor the magnifying power of a Microscope forms the standard of 

 its efficiency, and that amplifications of 2000 or 3000 diameters could 

 be obtained without any difficulty half a century ago, when, notwith- 

 standing, much less was visible than can now be seen with a tenth 

 of the power. 



In a subsequent paragraph it is stated that the " binocular form 

 of construction, though attempted very long ago, was not success- 

 fully carried out till 1851." 



* It must also be borne in mind that it is impossible to make reliable 

 observations as to the relative performance of objectives with different apertures, 

 unless the fact of their perfect correction is ascertained independently of the observa- 

 tions in question, that is on objects the correct appearance of which is not dubious 

 or hypothetical, as for instance, the outlines of thin silver films. 



Again, it is out of the question in such observations to make arbitrary 

 changes in the conditions under which the objective acts, as shortening and 

 lengthening the tube, interposing other lenses between the objective and the eye- 

 piece, using the objective with immersion fluids for which it was not con- 

 structed, &c. 



As wide apertures allow of much greater aberration than low ones, it 

 may happen that the former, if the correction is not very carefully made, will 

 show less than a low aperture, even if this is also badly corrected, because the 

 relative deterioration of the image is not so great. 



t ' A Popular History of Science,' by E. Eoutledge (8vo, London, 1881) 

 p. 515. 



% Dr. Goring suggested diamond lenses to A. Pritchard in 1824, and he made 

 one in the same year (see Sir D. Brewster's ' Treatise on the Microscope,' 1837, 

 pp. 13-21). Sir D. Brewster's reference to diamond lenses will be found in 

 4 Treatise on New Philosophical Instruments,' 1813, pp. 402-10 ; and ' Treatise on 

 Optical Instruments,' 1832, p. 39. 



