182 THE MICROSCOPICAL NEWS. 
fourth to be a homogeneous-immersion objectivé with cover correc- 
tion, and all to be of the highest possible grade of workmanship. 
The stand . . . to be furnished with six eye-pieces, viz. 2 in., 
1 in, and 3/4 in. Huyghenian, and 1/2, 1/3, and 1/4 in. solid. 
The following table shows the application of these powers to all 
grades of work, from that which is ordinarily done with a pocket 
lens to the extreme limits of microscopical vision :— 
Aiea 
eSod| = 
6 On| 8B 
i Beale How obtained 
ae |\N. A. required! Equivalent an- Po.S +B a iS Ot Oe 
Thar | to resolve. | gular aperture. Poe BS 
AO 8.4] 3 . 
Bo kn| 2S 
A0So|] 8a 
AA a. | 
| < a Objective. Eye-piece. 
100} Less than 0°10 Less than 10° air} None | None) Naked eye Naked eye 
500 Less than 0°10 Less than 10° air 5 123| 4 in. of 0:10 N.A. | 2 in. 
5,000) Less than 0°10) Less than 10° air 50 50 | Lin. of 0:°26 N.A.! 2 in. 
10,000) 011 12° 38’ air 100 100 te lin. 
20,000 OTs 24 16" 5, 200 200 Poe Zin. 
30,000 0°32, CAE ss 300 300 | 1/6in. of 0°94°N.A.| 2 in. 
40,000 0°41 48° 26’ ,, 400 600 He lin. 
50,000 0°52 62° 40’ ,, 500 600 Ae lin. 
60,000 0°63 73° 08’ ,, 600 600 she 1 in. 
70,000 0°73 93° 48’ ,, 700 800 A in. 
80,000 0°84; 104°17' ,, 800 800 ee 2 in. 
90,000 0°94 140° 16’ ,, 900 | 1200 eae $ in. 
180° air, 82°17’ 
96,000 1°00 (nomozeneous) 960 | 1066 | 1/8in. of 1°42N.A.| ¢ in. 
. imm. fluid 
100,000 1°04) 86° 21’ 1000 1066 | aae 3 in. 
110,000 115) About 98° ,, 1100 1600 585 34 in. 
120,000 1°25} About 110° ,, 1200 1600 Bar in. 
130,000 1°35| About 125° ,, 1300 1600 aie din. 
136,888 1°42) About 138° ,, 1368 | 1600 ase 4 in. 
It has not been my purpose to lay down any single set of 
objectives as the only proper one, but to indicate the principles on 
which selection should be made, and the relation of aperture to 
amplifying power, and to show that there is at present no good 
theoretical reason for the use of objectives of greater amplifying 
power than the 1/8 in.” 
Dr. Blackham, it will be seen, advocates the use of eye-pieces as 
high as 1/4 in. which is largely in excess of Prof. Abbe’s figures, 
which do not go beyond an amplification of 15 times.* 
Mr. J. D. Cox believes + “‘ Dr. Blackham has the verdict of ex- 
perience with him when he says four or five lenses with a proper 
number of eye-pieces will cover the whole range of microscopical 
examination. In such a number of lenses you may get all the 
necessary combination of the three qualities of angle, power, and 
* See this Journal, iii. (1883) p. 808. 
+ Proc. Amer. Soc. Micr., 6th Ann. Meeting, 1883, pp. 229-30. 
