ZOOLOGY AND BOTANY, MICHOSCOPY, ETC. 209 



usual formula 100 -!- 95000 x 0'13 = 1/123-5 in., which is at any 

 rate a safe limit above the conventional y^-^ in. 



The author also treats of the limit of useful mag^nification in con- 

 nexion with numerical aperture, of which he takes the highest attained 

 value, 1"50. Starting from this some authorities have thought that the 

 highest useful power will be 750, and that anything Ijeyond that value 

 would reveal no further structure. Other authorities have even quoted 

 577 as the limit. The author, however, considers that the Hmit of 

 useful magnification is given by the calculation 1*50 x 100 -=-0*13 ^ 

 1154, or even perhaps by 1'50 x 100 ^ 0*10 = 1500, and he quotes 

 the evidence of the photographs of Amphijjleura peUucida, given in 

 Spitta's Microscopy, as practical confirmations of his theoretical values. 



The author is of opinion that the yVi^- homogeneous immersion 

 objective usually sold cannot l^e improved upon, either in aperture or 

 in power. He concludes by advising a student to equip himself with a 

 low aperture for the usual f in. (IG mm.) objectives, one of N.A. 0*65 

 to 0*75 for the |-in. (4-2 mm.), and one of as high as he can afford 

 for the yV i^- (- i^^^- oil-immersion. From the last alone he will expect 

 the utmost resolution. The others will show him all that the eyes can 

 see without unduly forcing them by high oculars, and altogether he will 

 have a battery that will save his eyesight, his patience and his pocket, 

 and that will, above all, never disappoint him, or fail to show him all 

 that can be shown. The specialist may go a step farther and obtain 

 a TTT'iii- oil-immersion for the very highest power, not in place of the 

 yV-iii-? but to supplement it, and to prevent the use of too high an 

 eye-piece. 



Relation of Aperture to Power.* — E. A. Hutton's contribution on 

 this sulgect seems to have initiated a controversy. T. F. Smith, while 

 assenting to many of Hutton's remarks, takes exception to his advice to 

 restrict N.A. to the barely necessary limit. He thinks that there is an 

 advantage in an excess of N.A., and that it is a mistake to limit 

 eye-piece poAver to 10. A micro-objective with reserve of aperture is a 

 whole battery of lenses in itself, the progress from lower to higher 

 magnification being made by changing the eye-pieces instead of the 

 objectives. Five eye-pieces, ranging from four to twenty-seven powers 

 will give with a f-in. ol)jective on a 7-in. tulje magnifications of from 

 40 to 270 diameters. As regards a yVin. oil-immersion Smith has 

 never l^een able to work advantageously with anything higher than a 

 twelve-power eye-piece calculated upon a 10-in. tube. Hutton's remark 

 that even with a ten-power eye-piece many low-power objectives only 

 give a " foggy glare " unless the illumination is carefully attended to 

 is contrary to Smith's experience, who finds that his low-power objec- 

 tives only begin to do their work when under a twenty-seven eye-piece. 



As regards the increased resolution apparently ol)tained by micro- 

 photography, it will always be found that details in the photograph can 

 be seen visually in the image when looked for. 



Smith supports his views by micro-photographs, and considers that 

 Hutton has overlooked an important property due to reserve of X.A., 



* Knowledge, xxxvi. (1913) pp. 102-5 (4 figs.). 



