Royal Microscopical Society. 257 



a, h, the refracting portions of the ribs appear like elongated depres- 

 sions, c, having the shadows brought to the same side as in a. Again 

 deepening the focus without any other change, the elongated de- 

 pressed areas seem to interrupt the light in every direction, except 

 at their edges and oblique junctions, and thus map out the surface 

 into illusory dark cells with light borders, d. 



Changing the position of the scale and incident light, we find 

 the rib more highly refracting the hght at one end than the other, 

 furnishing the appearance of a clumsily-made bent pin, with faint 

 transverse markings still visible, e. The knobbed look is un- 

 doubtedly due, in part, to the rib being rather more elevated there 

 than in the other portions of its course, and corresponds to Mr. 

 Wenham's figure,* which is very truthful when an oblique view can 

 be obtained of the scale, and to the knobbed appearance of the notes 

 in Dr. Woodward's magnificent photographs ; also in Mr. Mclntire's 

 figure of Templetonia nitida.\ 



The interruption of the points of the pins seems to be chiefly 

 due to the depression of the rib at those parts from closer contact 

 with the opposite membrane, causing it also to bend a httle ofi" to 

 one side. 



Eetaining the illumination, but deepening the focus, we pro- 

 duce the figm-e/, still showing the transverse markings, the shape 

 closely corresponding with the figure 4 a, x 1250, pi. 7, engraved 

 in Quekett's work on the Microscope, second edition, where it will 

 be noticed the heads of the " notes " are rather notched than rounded, 

 as hkewise with very many of the " notes " in the beautiful recent 

 photographs. No. 346, new series, by Dr. "Woodward, of the " Test 

 scale," X 3200, received from him the 7th of this month (April), 

 and to whom I feel deeply indebted for the charming picture ; also 

 for a stereoscopic picture of the same scale, to hand somewhat later. 

 Although I can easdy combine most stereoscopic pictures without 

 the use of any stereoscope, I find it difficult to converge the eyes 

 sufficiently to close these images perfectly, yet when obtained, 

 though the " notes " form somewhat elevated areas above the other 

 surface, they have not the appearance of solidity one would at first 

 sight have expected ; but this leads me still more to hold the opinion 

 that their production is an optical efi"ect and not the structural ar- 

 rangement. The photo-micrographs are wonderful results of skill 

 in focussing the two different views, which scarcely depart perhaps 

 sufficiently from the optician's view, — to furnish a strong stereoscopic 

 image of the structure. The shadows also are more on one side, 

 and the central light is long and narrow. 



Still shifting the direction of the scale and the illuminating rays, 

 we produce much the appearance of the first focus, but with two 



* ' M. M. Journal,' No. XXI., p. 125. 

 t Ibid., No. XIII., Plate XXXVII. 



