1032 SUMMARY OF CURRENT RESEARCHES RELATING TO 



neck, which brings the eye-lens 12 to 15 mm. above the body. This, of 

 course, increases the optical tube-length just so much, and it would be 

 necessary for opticians to indicate on the objective whether it was corrected 

 for the Continental or the American ocular. With the measurement a to d 

 each microscopist could easily adapt his tube-length to suit either style of 

 ocular. 



We can join Prof. Gage also in his plea for ' par-focal ' oculars. Their 

 adoption would be another step in the development of a uniformity in 

 apparatus, which is of so great convenience to busy workers, and which 

 tends so much to harmonize the work of various manufacturers. 



We believe that these subjects, so tersely brought foward by Prof. Gage, 

 should be agitated until manufacturers adopt them ; and to further this end 

 we shall be glad to publish correspondence from all interested opticians 

 and microscopists." 



Measurement of Power.* — Mr. E. M. Nelson says that it is sometimes 

 useful to know the " initial " magnifying power of an objective, by " initial " 

 power meaning the size to which an image will be magnified by an objective 

 alone when projected on a screen at a distance of 10 in. 



In practically measuring this power, it will be found a more accurate 

 plan to increase the distance to, say, 60 in., and divide the result by 6. 

 These measurements are very easily performed when one has a camera, but 

 it is not so easy to do them without. Therefore, another and somewhat 

 loose way of getting at the initial power is adopted, viz. as follows : — 

 Measure the combined magnifying power of the objective, and say 2 in., or 

 A, eye-piece, and divide the result by 5. This method would do very well 

 if the exact multiplying power of the eye-piece was 5, and if the length of 

 the body remained constant. As it is not an easy matter to find out the 

 exact multiplying power of an eye-piece, Mr. Nelson recommends any one 

 desirous of knowing this to measure or get measured the initial power of 

 one of his objectives ; then measure the combined power of this lens and 

 his eye-piece, paying great attention to his tube-length during the operation. 

 This will give him once for all the multiplying power of his eye-piece with 

 that tube-length. He will tlien be in a position to ascertain the initial 

 power of any other lens with that eye-piece and the same tube-length. But 

 as the optical tube-length may differ from the actual tube-length, and does 

 differ to a certain extent, with objectives of ordinary construction, this 

 process is not so simple as it appears. In order to get fairly accurate 

 results with the higher powers, a certain percentage must be deducted. To 

 give some examples : 



Thus, 1 in. at 60 in. increases the image of • 01 in. to • 66 in., its power, 

 therefore, is 66, which at 10 in. = 11 = initial power. The combined 

 power of this lens with an A eye-piece is 55, which gives 5 as the multiply- 

 ing power of the eye-piece. Now, if the combined power of this eye- 

 piece with a 2/3 = 75 we may assume the initial power of the 2/3 

 is 15. 



If, however, we treat higher powers in the same way, we shall get too 

 high values. Thus the combined power of a 1/4 and the eye-piece is 203 ; 

 dividing by 5 we get 40*6 as the initial power, whereas 39 "3 is the real 

 power. 



Again, the combined power of a certain 1/12 and the eye-piece is 600, 

 which, divided by 5, gives 120 as its initial power, whereas it is in reality 

 113*2. The empirical rule Mr. Nelson employs is to deduct 2 per cent. 

 for 1/2, 3 per cent, for 1/4, 4 per cent, for 1/6, 6 per cent, for 1/8, 1/12, 



* Eng. Mech., xlvi. (1887) pp. 188-9. 



