PH0T0:^1ICR0SC0PE : J. H. G. MONYPENNY 147 



(d) The Relationship of AjJerture a7id Magnification. — With 

 the author's arrangement of condensers, the beam of light entering 

 the vertical illuminator is rather larger than the largest back 

 lens of any objective he has; it is therefore necessary to use the 

 iris diaphragm on the illuminator (D, Fig. 3). His practice is to 

 cut down as little as possible. Generally he leaves the back 

 combination 5-6ths full; it is only on rare occasions that he reduces 

 below this. If the aperture is cut down much more than this, any 

 surface irregularities due to scratches are shown up in a very 

 prominent manner, owing to diffraction bands. For this reason, of 

 course, it is well to reduce the aperture to less than 5-6ths when 

 any relief effects in the structure have to be emphasised. The effect 

 of gradually reducing the aperture of an objective has probably 

 been studied mostly from the point of view of the higher power 

 objectives. The bad effect produced on the images given by such 

 lenses owing to such reduction is probably well known, though the 

 fact that photomicrographs of metals showing diffraction effects 

 caused by such reduction are still published shows that this bad 

 effect is not always sufhciently appreciated. With lower power lenses 

 the effects are not so marked metallurgically, since, generally, the 

 photographs taken with such lenses give a general view over a large 

 field, and are not intended to show fine detail. In addition to this, 

 such low power lenses {e.g., 1 in. or 2-3rds) have in general a 

 higher ratio of N.A. to magnification than the higher powers. For 

 example, the N.A. of the lenses mentioned above is generally between 

 .24 and .30, and they are used for photographs at, say, 50 to 150 

 diameters. On the other hand, twelfths used at 1,000 and 1,500 

 diameters have at the most 1.4 N.A., and frequently only 1.2 to 

 1.3. Consequently there is more latitude with the stopping down 

 of low power lenses, but still it should be remembered that with 

 these lenses diffraction effects are produced, and there is a limit to 

 the reduction of the aperture beyond which it is not advisable to 

 go. By suitable stopping down, however, one can often, with these 

 low powers, obtain a larger field sharp all over — frequently of great 

 importance. 



Under present conditions nothing is gained by photographing 

 at any higher magnifications than about 1,500; with the present 

 maximum aperture available (N.A. 1.40), all detail which can be 

 shown is visible at this magnification. Any higher magnification 

 is of the nature of an enlargement, and can be obtained equally as 

 well by photographing at this magnification (x 1,500), and enlarging 

 from the negative, as by taking the negative direct at the higher 

 magnification. There is no doubt that for many metallurgical pur- 

 poses a higher magnification, coupled with greater resolvingf power, 

 would be of great value. This could be obtained either by using 

 light of very short wave-length, with its attendant difficulties of 

 focussing, and also the necessity of special lenses capable of trans- 

 mitting light of su/^h short wave-length or by increasing the aperturo 

 of the objective. With regard to the latter, the author believes 

 the firm of Zeiss produced some years ago a 2.5 mm. objective 

 working at about N.A. 1.65. This objective had a front of flint 

 glass, and used as immersion fluid monobromide of naphthaline. 

 Its use, however, for transparent work was attended with great 

 difficulties and expense, inasmuch as the slip and cover glass had 



