ZOOLOGY AND BOTANY, MIOEOSOOPY, ETC. 333 



be the guide in this. A picture is not spoilt by being exposed even 

 twenty times too long. Development is best done while looking 

 through the plate, and as soon as the speck containing the picture 

 distinctly appears, wash off the developer, and fix. A good magnifier 

 will then show how much success has been obtained ; if worth 

 examining more critically, a second Microscope is a luxury, but, 

 unless the magnifier shows a good image, it is certainly not worth 

 while to disarrange the Microscope to examine it, if only one is in 

 the operator's possession — especially as another plate can be exposed 

 in two or three minutes. In fact, by having a dipper to hold half-a- 

 dozen plates, they can be exposed, developed, and fixed in about as 

 many minutes. 



When success is fairly attained, a special negative may be taken 

 for reduction, and a mask used to cut off all not desirable to appear 

 in the tiny positive. It is needless to say, that this negative must be 

 very sharp, and as clean as possible. 



Instantaneous Photo-micrography.* — Mr. D. S. Holman has 

 recently made some experiments in photo-micrography. Having 

 succeeded in taking photo-micrographs of rapid vibrations, he deter- 

 mined to attempt to photograph Amoeba proteus and other low forms 

 of life while in motion. His method was as follows : — 



Having inclosed the material in one of the Holman life-slides, 

 and allowed it to remain until the Amoehae had become accustomed to 

 their new home and active, he cast an image of an Amoeba on the 

 ground glass of a camera, by means of a Holman lantern Microscope, 

 which is illuminated with the oxy-hydrogen light. A Zentmayer 1/5 

 objective was used. A dry plate picture was then taken with about 

 one-hundredth of a second exposure. Two exposures were made of 

 one Amoeba at intervals of three minutes, and one exposure of two 

 Amoebse in the field at one time. The photographs were a complete 

 success, and were shown at a recent meeting of the Franklin Institute 

 magnified 10,000 diameters, making a picture of about eight feet on 

 the screen, so accurate that the granular appearance of the proto- 

 plasm could be distinctly seen. 



" On the possibility of improvement in the Microscope."t — Dr. 

 E. Altmann discusses the directions in which further improvements 

 in the Microscope are likely to be made. 



The absolute efficiency of an objective being E = , where 



2 sin a 

 a = semi-angle of aperture and A, = wave-length of the light 



employed, then in the most favourable case possible (a = 90°) E = - • 



The value actually attainable is limited, by difficulties of correction 



to an angle of aperture of 120° corresponding to E = —. 



The improvement theoretically possible by increase of aperture is 



* Sci.-Gossip, 1886, pp. 43-4. 



t Arch. f. Anat. u. Physiol. (Anat. Abtheil.), 1886, pp. 64-8 (2 figs.). 



