693 



OBITUAKY. 



Francis H. Wenham, C.E. 



Vice-President Royal Microscopical Society, 1870-1, 1873-4. 

 Died August 11, 1908. Aged 85. 



Mr. Wenham was the originator of numerous mechanical inven- 

 tions, one of which, his inverted Argand gas-burner, will be familiar 

 to most of us. He devoted about thirty years of his long life to 

 microscopical work, and it is with this portion of his activities 

 that the Fellows of this Society are most concerned. We first meet 

 with him in 1850, when he brought before this Society a metal 

 parabolic reflector ; one of these very rare pieces of microscopical 

 apparatus, the first made, is in our cabinet of ancient instruments. 

 In his second paper he described a binocular ; this he subsequently 

 improved in 1860, and later in that year he brought out the ortho- 

 stereoscopic binocular, which is still used, and is the best that 

 has ever been designed. In 1873 he designed a high-power non- 

 stereoscopic binocular, which did not come into general use. In 

 the Quarterly Journal of Microscopical Science, ii. (1854) Wenham 

 published a paper on " The Theory of the Illumination of Objects." 

 A perusal of this in the light of modern knowledge is of interest. 

 On page 146 the author says : — " Attempts have sometimes been 

 made to draw the undulatory theory of light into the subject of 

 microscopic illumination, but without any substantial reason, as it 

 has in reality very little or nothing to do with it." Again, on 

 page 1 52, he says : — " There is one peculiar phenomenon attendant 

 upon oblique illumination at certain angles in one direction, and 

 may be described as a double image, or kind of overlying shadow, 

 having in some instances markings equally distinct with those of 

 the object itself. This appearance has been termed the ' diffract- 

 ing spectrum ' among men of science. Taking the name to be 

 descriptive, I sought for an explanation in the known laws of the 

 diffraction of light, but these did not account for it, for on this 

 theory I attempted to find the clue in vain. I have since traced 

 the cause entirely to the mutual dependence of the angles of illu- 

 mination and aperture, detailed in this paper. One image is caused 

 by the radiations from the object entering one portion of the object- 

 glass, and a different one by the object being directly seen by in- 

 tercepted light with the other extreme of the aperture, thus 

 giving the appearance of a double image. In proof of this, hold 

 a card over that side of the front lens of the objective which 



