490 SUMMARY OF CURRENT RESEARCHES RELATING TO 



objective, though it is still an open question whether better negatives can- 

 not bo produced without its use, for the dangers of absolutely correct 

 centering are very great. He also appears rather to prefer the use 

 of the old term microphotography, but he certainly acted wisely in 

 adhering to what is now the standard and well recognised term which, 

 however imperfect, has been admitted since 1864, if not earlier, although 

 the fatherhood has been made somewhat doubtful by the impossibility of 

 finding any printed record of its first use. It has, however, been so gene- 

 rally accepted by the foremost workers since that time, that no other 

 nomenclature can now take its place. Macrophotography was proposed 

 many years since, but never found favour, for it would rather apply to 

 reasonable enlargements, whether from photomicrographs or ordinary 

 negatives, than to the photographic image produced by the Microscope in 

 the first instance. 



The ijhotographic use of the new " Apochromatic " objectives with their 

 accompanying "projection" eye-pieces receives a favourable notice, and 

 theory is certainly in their favour. 



There are some points in this manual which are a little dogmatic, and 

 others which may be enlarged upon in future editions with advantage to 

 the beginner. The retention of the ordinary brass photographic mount, the 

 lenses being removed, is very questionable unless the screw rims be j)er- 

 fectly blackened, and then if there be a central diaphragm the field may be 

 too much limited. The attempt to photograph ditferent planes by succes- 

 sive focusing, however i^erfect the fine-adjustment, is open to question, for 

 the different photographed planes when developed must overlie each other 

 and tend to confusion, except with very simple objects. It has been usual 

 to find the most perfect visual focus of whichever plane gives the truest 

 aspect of the whole, and to photograph that, using a rather slow plate, 

 a low angle objective, full exposure, and slow development well restrained. 



HiTcucocK, K. — Photomicrography. IX. 



[Sensitizing the paper, priuting, mounting, &c.] 



Amer. Mon. Micr. Journ., VIII. (1887) pp. 41-1. 



(5) Microscopical Optics and Manipulation- 

 Magnifying Power of Dioptric Instruments. — M. A. Guebhard * has 



cleared up the disagreement which appeared to exist between theory and 



practice in regard to magnifying power. 



Magnifying power involves a comparison between the apparent size of 



an object seen with and without the optical instrument, by apparent size 



being meant the size of the image on the retina. This is proportional to 



the visual angle (or its tangent), that is to say, it may be measured by ^ 



where h is the absolute size of an object and d its distance from the first 

 nodal point of the eye. The apparent size may therefore be increased 

 indefinitely by bringing the object near the eye, until the punctum proximum 

 or least distance of distinct vision is reached. Defining then the magni- 

 fying power as the ratio of the visual angles under which the object is seen, 

 with and witliout the instrument respectively, when the conditions are as 



* Rev. Scientif., 1883, pp. 804-11 (5 figs.). Transl. by G. Fisclicr, Centml-Ztg. 

 Oi)tik u. Mecb., v. (1884) pp. 183-8 (G iigs.), 194-7. Cf. also pp. 217-20 (3 figs.). 



