THE AMERICAN 



MONTHLY 



MICROSCOPICAL JOURNAL 



Vol. V. "Washington, D. C, February, 1884. No. 2. 



Optical Tube-length and Magnifi- 

 cation. 



The last number of the Journal 

 of the Royal Microscopical Society 

 contains an article by Mr. Frank 

 Crisp the Secretary of the Society, 

 which shows that the great majority 

 of microscopists have held erroneous 

 opinions concerning the relations of 

 tube-length and the amplification of 

 the optical combination in a micro- 

 scope. The meaning of ten-inch tube 

 has not been accurately defined, so far 

 as we are aware, in any English text- 

 book on the microscope. In practice 

 some persons measure the tube itself, 

 others say it should be the distance 

 from the "^ optical 

 centre" of the ob- 

 jective to the top 

 of the tube, others 

 again measure 

 from the dia- 

 phragm of the ocu- 

 lar. The whole 

 matter is in a state 

 of utter confusion, 

 and since the com- 

 mittee of the Ame- 

 rican Society of 

 Microscopists has 

 likewise been in 

 the dark about the 

 matter, as shown 

 by the report in 

 which the ten in- 

 ches is measured 

 "from the dia- 

 phragm of the ocu- 

 lar to the front lens 

 of the objective," 

 the article by Mr. 

 Crisp comes in good time. 



For the complete consideration of 



Fig 



this subject the reader is referred to 

 the original article, but sufficient will 

 be given in this place to make the sub- 

 ject clear. 



It will be found in practice that : — 

 "i. Two objectives of precisely the 

 same focal length, used with the same 

 tube and the same eye-piece, may 

 nevertheless give different magnifying 

 powers. 2. Tw^o objectives of differ- 

 ent focal lengths, used with the same 

 tube and eye-piece, will not give mag- 

 nifying powers in proportion to their 

 focal lengths ; thus a ^-inch will not 

 necessarily give double the power of 

 a I -inch. 



"Conversely, two eye-pieces will 

 not amplify in proportion to their fo- 

 cal lengths, though used with the 

 same tube and objective." 



The true magnification may differ 

 by IOC per cent, from the power cal- 

 culated upon the ordinary assump- 

 tions. The explanation is found in 

 the erroneous notions concerning tube- 

 icngth. 



The magnification of a lens is de- 

 termined by its focal length. It is 

 measured by dividing ten inches, the 

 distance of distinct vision, by the fo- 

 cal length. Lety"= focal length, / = 

 ID inches, M = magnifying power. 



/ 

 Then J/= j . For a >^-inch, M = 



lo ^ ^ ^ So diameters. The mag- 

 nifying power of an objective is im- 

 mediately given by multiplying the 

 denominator of the fraction express- 

 ing its focal length by lo. Thus a \ 

 magnifies 50, a jt, 60, if the image be 

 received on a screen 10 inches from 

 the posterior focal plane of the lens. 

 For microscopical use it is neces- 

 sary to extend this formula to the com- 



