Royal Microscopical Societtj. 267 



Note on the ahove. By F. H. "Wenham. 



In inadvertently making use of the term " shown opaquely," 

 I did not wish it to be inferred that I considered this method as 

 strictly an opaque illumination, which is understood when the light 

 is thrown only on the upper surface of the object. 



The truncated lens, or flat-topped parabola, first used by me in 

 the way referred to above, gives such a briUiant luminosity to the 

 object, on a jet-black field, that it has all the appearance of an 

 opaque illumination, and perhaps on many objects the difference 

 in apparent structure would not be material, and may be illustrated 

 in this way : Suppose some semi-transparent body, such as a green 

 grape, be let into a piece of black card ; on holding this against 

 a strong hght, so that it enters sideways, the seeds and internal 

 structure will be shown satisfactorily. If a side light is condensed 

 down upon the object, the same internal structure will be seen, though 

 not so perfectly on account of surface glare. When a side Hght 

 is thrown into the body of an object either way, each dense particle 

 that intercepts it serves to illuminate its neighbour, and so the rays 

 are diffused in every possible direction, and if the structure contains 

 particles actually impervious to light, they will not be seen hke 

 dark shadows as by dii'ect light, but luminous, and ia their natural 

 colours. I consider this is the main principle — to send the light 

 into the object in any or all directions beyond the angle at which 

 rays from the source can enter the eye. 



Dr. Woodward has kindly sent me the photographs referred to 

 in the above note. No. 2, which quite agrees with my description, 

 is in places very sharp and distinct, showing the intercostal striae 

 or bars plainly. Nos. 1 and 3 are somewhat blurred, and to my 

 mind do not show structure satisfactorily. Knowing the difficulty 

 of obtaining a photograph of an object of this character, merely 

 fi-om its own diffused light, I was much surprised at Dr. Woodward's 

 remarkable skill in producing a perfect picture — a feat that I should 

 have thought scarcely possible. 



