THE QUALITIES OF OBJECTIVES 425 



power is very significant, and should be carefully pondered by those 

 who still desire low apertures as the only perfect form of objectives. 



It is as well to mention that objectives may be arranged in two 

 series one the 2, 1, ^, J, and , the other 1J, J, J, ^, ^. One of 

 these series will form a complete battery, as it is unnecessary to 

 have objectives differing from the next in the series by less than 

 double the power. 



The most usual combination is perhaps the 1 and the J of one 

 series, or the and the of the other. Of these two preference 

 might rather be given to the latter. The only exception would be 

 the addition of a 1 J-inch for pond life. 



Eye-pieces should also double^ the pow y er thus : 5, 10, and 20 

 (imcompensated), or 6, 12, and 27 (compensated), the most useful of 

 the three being the 10 (uncompensated) and the 12 (compensated). 

 As there is no 6-power compensated eye-piece for the long tube, a 4 

 for the short tube admirably answers the purpose. 



In addition to the explanations already given on the subject of 

 testing objectives, it may be useful here to note that the qualities of 

 an objective are seven in number : 



1. Magnifying power (initial). 



2. Aperture or N.A. 



3. Resolving power. 



4. Penetrating power. 



5. Illuminating power. 



6. Flatness of field. 



7. Defining power. 



1 . Magnifying power. No test is required, as the initial magni- 

 fying power can be directly measured. 



2. Aperture or N.A. can be directly measured ; no test is there- 

 fore necessary. 



3. Resolving power. A lens illuminated by a large solid axial 

 cone, when a Gifford's screen is used, should resolve a number of 

 lines to the inch expressed by its N.A. multiplied by SOjOOO. 1 



4. Penetrating poiver is the reciprocal of the resolving power of 



^- . . No test needed, but penetrating power varies largely with 



the combined magnifying power, and also with the magnitude of the 

 illuminating cone used, as already intimated. 



5. Illuminating poiver is the square of the numerical aperture 

 (N.A.) 2 . No test is necessary, but the remarks made above in 

 regard to penetrating power apply equally here. 



6. Flatness of field is, in the strict meaning of the term, an 

 optical impossibility. The best thing therefore is to contract the 

 visible field, as is done in the compensating eye-pieces. (Tests : For 

 low powers a micro-photograph ; for medium and high powers a stage 

 micrometer.) 



7. Defining power depends on (a) the reduction of spherical 

 aberration, (b) the reduction of chromatic aberration, (c) the perfect 

 centring of the lenses by which is meant (i.) the alignment of 



1 J.B.M.S. 1893, p. 15. E. M. Nelson. 



