274 E. M. NELSON ON A PHOTO-MICROGRAPHIC PHENOMENON. 



light from a condenser, for example the P. angulatum.* When a 

 suitable diaphragm is placed at the back of the condenser we get 

 the dioptric beam surrounded by the six well known spectra. Now 

 we know that if we place at the back of the objective a stop 

 cutting off the six spectra, on looking at the image we shall see a 

 P. angulatum without markings on a bright field. Suppose, now, 

 that we photograph this and print a lantern positive. Next let us 

 stop out the central dioptric beam and admit the six spectra. We 

 shall now have an image of the P. angulatum resolved on a dark 

 ground. Let this be photographed and a lantern slide made from 

 the negative. 



If we were to place each slide in a lantern and superimpose the 

 images on the screen, we should find that the high lights in the 

 diatom would be brighter than the field, because they have received 

 strength from the second image which the field has not. This is, 

 I believe, the true explanation of this very curious phenomenon. 

 Whilst on this subject let me remark that I believe it to be a 

 physical impossibility to obtain a black background in a negative 

 of the critical image of a diatom. My experience is that the more 

 critical the image the less the contrast. So long as you are con- 

 tent with a lot of white beads obtained by stopping down your 

 condenser, putting it out of focus, or other means, it is comparatively 

 easy to get a considerable amount of contrast. But the moment 

 you seek a really critical image, and put your objective on its 

 mettle by enlarging your central illuminating cone, so soon do you 

 lose contrast. 



Further, I am strongly of opinion that a print, to be of any 

 scientific value, must bo taken from an untouched-up negative. 

 Intensification is also to be deprecated because it gives a false con- 

 trast, such as is not to be obtained when viewing the object through 

 the microscope in the ordinary way. In brief, a photo-micrograph 

 should be an exact representation of an object as we see it, and 

 not a picture of our imagination. Some have brought forward 

 photo- micrograph s as evidence that their particular view of an 

 object must be the correct one. This does not at all follow, as one 

 can obtain in a photo-micrograph as great a variety of appearances 

 as can be seen by the eye through the eye-piece. 



* This, while suitable for an example because its six spectra are generally 

 known, dues not exhibit the phenomenon as strongly as some other diatoms. 



