ZOOLOGY AND BOTANY, MICROSCOPY, ETC. 679 



3. 501 lines '01 mm., and 501 lines ^sV^ inch. 



4. 1001 lines '01 mm,, and 1001 lines -^rioo inch. 



5. The same as the preceding one, but with both sets of lines 



double ; that is, ruled both with fine and with coarse lines. 

 In all the above, the 5th and 10th lines are longer than 

 the others. 

 The object in ruling the lines 2^^^ of an inch apart is to permit 

 of ready comparison with the • 001 mm. lines, these spaces being ap- 

 proximately equal. It will be seen that one band acts as a vernier to 

 the other. 



The extreme working length of the screw of the machine is half a 

 metre. The theoretical limit of subdivision is about two billionths of 

 a centimetre. The practical limit may be set at about one fifty- 

 thousandth of a centimetre. 



Ideal Series of Objectives for Microscopical Work.* — Governor 

 S. D. Cox, of Cincinnati, suggests the following as what might fairly 

 be called an ideal series of lenses : — 



" (1) An objective of 40° aperture and half an inch working dis- 

 tance, giving about 40 diameters magnification with the ordinary No. 1 

 ocular, and resolving 38,000 lines to the inch. (2) An objective of 

 100° aperture and one-eighth of an inch working distance, giving 

 about 120 diameters, and resolving 70,000 lines to the inch. (3) A 

 homogeneous-immersion objective of 120° balsam-angle of aperture, 

 giving about 300 diameters, and resolving 120,000 lines to the inch. 

 Proper eye-pieces would make these three objectives cover the interme- 

 diate magnifications desirable, and the third objective in the list would 

 resolve any test resolved by any glass yet made and in the market, 

 whilst the 40° glass would give all the ' penetration ' needed for the 

 binocular with opaque objects." 



Dr. J. Edwards Smith also recommends | the following series of 

 objectives as best fitted for microscopical work chosen from the stand- 

 point of a protracted experience " over the tube " : — • 



First, a |-inch, aperture 45° to 47°, having a working distance of 

 ^ inch, thus suitable for wet mounts without covers, and resolving 

 35,000 to 40,000 lines. This comes tolerably close to the ideal inch 

 of 40°, both as to resolving power and working distance. 



Second, a " real good honest " ^ inch of 40°, recommended as a 

 " hack " and for work over acids, &c. 



Third, a first-class wide-angled duplex objective, say a ^ or a 

 Y^Q-, resolving all the most difficult tests. 



High Amplifications. | — Eeferring to the note at pp. 127-9 of this 

 volume. Dr. Phin disclaims the idea of being an advocate for excessive 

 amplifications ; on the contrary, he has always opposed their use. " It 

 " is now pretty well recognized that very high-power objectives do not 

 " reveal anything more than those of moderate power, it being assumed 



* ' Cincinnati Medical News,' Jan. 1S81. 

 t Amer. Journ. Micr., vi. (1881) pp. 66-7. 

 % Ibid., p. 64. 



