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On the Interpretation of a Photo-micrographic Phenomenon 

 by the Abbe Diffraction Theory. 



By E. M. Nelson. 



{Read May 25th, 1888.) 



Most photo-micrographers have observed that what in photo- 

 graphic language may be termed the high lights in a diatom, print 

 darker in the negative than the ground or field. This is especially 

 remarkable when my method of obtaining a critical image is 

 employed, viz., that of viewing the object in the image of the 

 source of light formed by a substage condenser. 



One would be much astonished if on interposing a medium, how- 

 ever thin, between one's self and a source of light (e.g., a piece of 

 glass before a white cloud) the light appeared brighter through 

 the medium than without it ; for we cannot conceive of any medium 

 that would not stop some light. With the microscope, however, 

 such is not the case. When a diatom, for instance, is placed in 

 an image of the source of light, the clear parts or interspaces of 

 the diatom are brighter than the field. I first noticed this on my 

 attempts to get density of background without over-exposure of 

 image. And since, when employed in ordinary microscope work, I 

 have particularly noted the relative brightness of field and inter- 

 space, and have found that the image received by the eye exactly 

 corresponds with the picture impressed on the plate, viz., the high 

 lights in the diatom are brighter than the field. 



It must be remarked that a very slight difference in visual 

 intensity when looking through the microscope will be sufficient to 

 cause a marked contrast on the photographic plate. 



It is probable that no one would have observed this small 

 difference in visual intensity unless they were specially looking for 

 it. 



The question naturally arises, How is it caused ? and I think 

 that an answer will be found in a careful study of the diffraction 

 theory. 



Let us in the first instance take a diatom, and view it by central 



Journ. Q. M. C, Series II., No. 22. 21 



