296 EDWARD M, NELSON ON 



reduced to 20 minutes there will be a great gain, not only of time, 

 but also in saving of fatigue to the observer. 



One more point and I have done. It is on a subject intimately 

 connected with the objective, viz. the measurement of the 

 magnifying power, and this is also in a deplorable condition.^ It 

 is the desire of some to dethrone English systems and to replace 

 them by German methods. Now, there are two systems for the 

 measurement of magnification in use in Germany, but no reason 

 has been put forward why one method should be chosen in 

 preference to the other, and, of course, the wrong one has been 

 chosen. The Zeiss method, the inferior of the two, uses the 

 initial power of the objective, but gives to the eyepiece power 

 adjusted to the tube length ; while the Leitz system more scien- 

 tifically gives the initial magnifying power to the eyepiece and an 

 adjusted one to the objective. If you will consider for a moment 

 the optical conditions you will see that the front foci of the eye- 

 pieces lie at a fixed point, whereas the back foci of the objectives 

 travel up and down the tube according to their powers and 

 constructions. All know that a low-power achromatic lens has its 

 back focus high up the tube, thus shortening the optical tube 

 length and so reducing the power. It is therefore a function of the 

 objective that reduces the power and not that of the eyepiece, 

 consequently the objective ought to have the adjusted power and 

 not the eyepiece. Here follow examples of each. Ist-Zeiss 

 method : No. 1 eyepiece, focus 50 mm. power adjusted to 

 150 mm. tube, x 3. Objective 36 mm. initial magnifying power, 

 X 7. Combined power, 21. 2nd-Leitz method with the same 

 eyepiece and objective : Initial power of eyepiece x 5. Power 

 of objective adjusted to 150 mm. tube, x 4:"2. Combined power 21, 



^ Sxirely " deplorable " is not too strong a term to tise ; for example, 

 there are four modern 1/2-in. object glasses on my table, and here are 

 their measured values : 



The ntunerical apertures vary, so that some are nearly twice as much 

 as others ; and one 1/2 in. is almost a 4/10 in. , and another is very nearly 

 a 1/3 in. ; the optical indices vary from 16 to 23. 



