192 PEACTICAL PHOTO-MICKOGRAPHY 



microscope projected exactly on the centre of the ground-glass 

 focussing-screen. It may happen that the particular point 

 in an object that is required to be shown up, even at the 

 expense of other parts, does not lie exactly at the centre 

 of the field, and in this case of course the above rule will 

 have to be departed from. But in general it will be found 

 more satisfactory to focus one central point in the object 

 and allow the remainder to take its chance. This applies 

 particularly when using apochromatic objectives, since unfor- 

 tunately their field is small ; it therefore becomes more 

 important that the essential part of the image should be 

 central rather than to attempt to focus any zone that lies 

 away from the centre. 



The apparatus described on p. 119, as devised by Mr. J. W. 

 Gordon, provides that when the object is visually in focus, the 

 camera has only to be placed in position and the photograph 

 taken, when a perfectly defined photograph results ; in other 

 words, no additional focussing is required between the obser- 

 vation of the image and the actual process of obtaining the 

 photograph. While this is unquestionably extremely con- 

 venient where only records of researches are desired, and not 

 highly critical photo-micrographs, it is, however, a method 

 that does not lend itself to very general use. In Mr. Gordon's 

 arrangement the photographs are quite small; in fact, the 

 sensitive plate occupies a position very close to the top of the 

 microscope tube, so that magnification is obtained entirely 

 by combination of objective and ocular. 



The application of this method to general photo-micro- 

 graphic work was described as long ago as 1902. l It is effected 

 by substituting for the ground-glass screen a minus spherical 

 lens, and using this for focussing purposes. Such lenses may be 

 obtained from an optician, and they are usually supplied in a 

 series from 1 D to 12 D. The lens that should be used for 

 a particular magnification depends upon the eyesight of the 

 operator, and the selection of the particular lens for a given 

 magnification must be determined by taking a photograph. 

 Instead of focussing on the ground-glass screen, the spherical 

 lens is placed over the ocular, and the image is focussed 

 through this lens. The latter is removed before exposure the 



1 Journal of Applied Microscopy, pp. 2082-84. 



