68 



THE AMERICAN MONTHLY 



[April, 



is not sharp, try changing the focvis 

 until a satisfactory result is obtained, 

 and note the correction required for 

 all subsequent work. 



It is by no means an easy matter to 

 focus a delicate microscopic object on 

 the ordinary ground glass of a camera, 

 because the grain of the glass inter- 

 feres, particularly when a magnifying 

 glass is used. To overcome this 

 difficulty, several methods may be 

 adopted. Perhaps the simplest is to 

 substitute a plate of plain glass for the 

 ground glass, and mount a simple lens 

 as a focussing glass, with its focus 

 adjusted to the plane of the surface 

 upon which the image is received. 

 An ordinary bank-note ' detector will 

 serve the purpose perfectly well. In- 

 stead of removing the ground glass to 

 substitute a plain one, the latter may 

 be used in an ordinary plate-holder. 



An excellent plan was described 

 two years ago in these columns by 

 Mr. George O. Mitchell.* Thi'^s 

 device consists of a strip of wood car- 

 rying an ordinary eye-piece. The 

 focussing screen is removed, and the 

 eye-piece adjusted to receive the 

 image while the strip of wood rests 

 across the end of the camera. First 

 focus an object on the ground-glass, 

 then remove it and adjust the ocular, 

 by sliding it out or in, until the image 

 is sharp. The device is then ready 

 for use. If it be found that the pic- 

 ture is not sharp, although carefully 

 focussed, because of the reasons 

 already mentioned, it will be advis- 

 able to change the position of the 

 ocular so that when the object is 

 sharply focussed for the eye it will 

 give also a sharp photograph. 



Having focussed the object the 

 sensitive plate may be introduced 

 and the exposure made. In filling 

 the plate-holder in the dark room 

 the plates should be removed from 

 their box, held in the left hand, being 

 careful not to touch the sensitive sur- 

 face v^dth the fingers, and the surface 

 lightly brushed with the soft brush 

 to remove particles of dust which 



* Vol. V, p.8i. 



S 

 _!_ 

 1 



would show in the picture. Place 

 them immediately in the holders, 

 and as a precaution, when working 

 by daylight, wrap the latter in a dark 

 cloth, as some plate-holders are not 

 absolutely light proof. 



As regards the length of exposure 

 it is almost useless to give any in- 

 structions, as this must be learnt by 

 experience. Mr. Walmsley gives 

 the following times of exposure for 

 his apparatus, which may be sug- 

 gestive to beginners using lamp- 

 light : — 



i^ inch, 3 to 45 seconds. 

 1^ "• ;J to i^ minutes. 



10 2 ^'-' 6 



2 to 7 " 

 5 to lo " 



In using sunlight the exposures 

 must be made exceedingly short. For 

 objectives lower than a 4-inch a me- 

 chanical shutter is almost indispensa- 

 ble with bright sunlight, for an ' in- 

 stantaneous bv hand' exposure would 

 spoil the plate with too much light. 

 Still, this depends upon so many 

 conditions that it is almost useless to 

 say this. Experience alone can 

 teach the proper exposure. 



4. Developing. — Before attempt- 

 ing to develop a plate the beginner 

 will do well to refer to the remarks 

 on page 203 of the preceding volume, 

 briefly explaining the chemical op- 

 erations involved. 



A word of caution to the amateur 

 photographer may not be amiss. Not 

 everything that is published, even in 

 the journals devoted to photography, 

 can be accepted without question. 

 The practical photographer may well 

 smile at the impractical schemes and 

 devices of the amateur, his wonder- 

 ful achievements with new developers 

 of complex composition, and his re- 

 markable discoveries of the effect of 

 mici'oscopic quantities of various 

 inert chemicals in the developer. 

 It is safe to say that a large propor- 

 tion of photographic literature is very 

 useless reading. It is rarely that the 

 new developers, that are constantly 

 being brought forward, are in any 



