2082 Journal of Applied Microscopy 



LABORATORY PHOTOGRAPHY. 



Devoted to Methods and Apparatus for Converting an Object into an Illustration. 



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FURTHER NOTES ON A NEW METHOD OF FOCUSSING 

 IN PHOTOMICROGRAPHY. 



At the request of the editor, we have prepared the following notes on our 

 method of focussing, described in full in the Zeit. f. Wiss. Mik. Band 18, 1901 : 



The method offers special advantages for the vertical camera ^ and daylight 

 illumination, as it does away with the use of the ground glass, a minus spherical 

 lens being substituted for the purpose of focussing. These lenses can be obtained 

 from any optician, and a series (omitting the half numbers) ranging from — l.D. to 

 ■ — 12. D. will furnish the equipment necessary for photographing at 12(10 diameters, 

 or less, with most combinations of objective, eyepiece and bellows draw. The 

 lens for a definite magnification depends upon the eyesight of the operator. The 

 selection of this lens is a simple matter, and can be determined by taking one 

 photograph. ^ 



Our method in brief is as follows: Instead of attempting to focus on the 

 ground glass fine details impossible to see with daylight illumination, the change 

 of focus necessary to throw the exact image (selected for the photograph) on the 

 ground glass, is accomplished by focussing through a minus spherical lens placed 

 on top of the projection ocular. This lens is removed before the plate is 

 exposed — \\\& photograph is not taken through the lens. The use of these lenses 

 is simply a device for compelling the eye to see the plane of the preparation that 

 is projected on the ground glass. 



For example, in photographing a centrosome at 1000 diameters (oil immer. 

 lens, projection ocular 4, diaphragm at 0), if an operator with normal eyesight 

 takes a sharp focus on the centrosome through the ocular, and exposes the plate 

 7vithout changing the focus, the negative will show the centrosome very much 

 out of focus; indeed, if the centrosome is small it will not be reproduced on the 

 negative at all. The sharpest details on the negative will be those on a plane 

 of the preparation lower than the one seen through the projection ocular. 



To get the correct focus for the negative, place a — <).D. spherical lens (assum- 

 ing that this is the right number for the operator) on top of the projection ocular. 

 Through this lens the eye will see the plane of the preparation that is shown in 

 the negative just taken, and it will be necessary to raise the focus to get a sharp 

 focus on the centrosome, the plate requiring a higher focus than the normal eye. 

 Having secured an accurate focus on the centrosome through the — 6.D. lens, 

 remove the lens and expose the plate. The negative will now show the centro- 

 some clearly as seen through the — tJ.D. lens. 



1 We use the Bausch & Lonib vertical camera, with the addition of a time shutter. With day- 

 light illumination the Carbutt, backed, ortho-portrait (27) plates give very satisfactory negatives 

 with three or four minutes exposure, for 1000 magnification, using an oil immersion lens. 



-See Zeit. f. Wiss. Mik. Band iS, 1901. 



