part 3] AND STEFCTUEE OF THE rOEAMINIFEEAL SHELL. 201 



of two adjacent arches, it then broadens out as it continues 

 downwards through the underljdng pillar, and finall}'^ narrows 

 again as it proceeds through the crown of the arch upon which 

 the pillar stands. The curved arms are given off, one on each 

 side of the base of the pillar, and follow the curve of the arch 

 along its upper margin. On rotation of the stage these arms 

 remain extinct; and with a rotation of 45° they form with their 

 fellows in the same zone a continuous wavy band, which follows all 

 the undulations of the arches. 



In the vertical bar we have straight extinction of horizontal 

 fibrils above and below and of vertical fibrils in the middle ; in 

 the curved arras the optic axes must be standing vertical to the 

 plane of the section and transverse to the length of the fibrils. 



It may be observed further that the dark areas in the figure 

 just described are not uniforml}' black, but splashed by illuminated 

 dots and dashes, and conversely the illuminated areas are speckled 

 wdth dark dots and dashes : an appearance which indicates the 

 existence of fibrils crossing the direction of their fellows, and as 

 well of variously orientated granules. 



In this felt-like structure we find an explanation of the por- 

 cellanous character of the Imperforate shell : the test is materially 

 continuous but optically discontinuous, or at least heterogeneous, 

 and tlie entering light, repeatedly refracted and reflected, loses 

 itself in reverberations. Thus the shell is white for the same 

 reason that snow is white. So far as keratin is present it will 

 contribute to this effect. 



But, again, may not the honey-^^ellow colour seen in thin 

 sections by transmitted light be a related phenomenon, the effect 

 of a turbid medium ? On this question being put to my friend, 

 Prof. Lindemann, he thought it not imlikely, and suggested a 

 comparison with the sky. Afterwards, when examining under the 

 microscope one of my sections which had always puzzled me by 

 appearing blue instead of yellow, I tried the effect of shading off 

 the light falling upon it from above. The blue then disappeared 

 and the usual honey-yellow was seen by transmitted light. On 

 cutting off the transmitted light and viewing it by reflected light 

 alone, the blue reappeared in greater purit3\ Thus the comparison 

 would seem to hold, and the shell is yellow by transmitted light 

 for the same reason as the sk}^ is blue by reflected light. ^ 



It ma}^ next be asked whether particles sufiiciently small and 

 numerous to produce this effect are present in the substance of 

 the shell. 



Here allusion may first be made to some observations by 

 Awerinzew, who, after heating some Orhitolites for two or three 

 minutes in a bath of fused potassium iodide (which melts at 



^ The phenomenon is a very common one ; I have observed it in chalcedony 

 both before and after heating- to redness, to a less degree in gypsum similarly 

 heated, and to a still higher degree in a film of balsam which had been spread 

 on a glass slide and exposed to the action of fluoric acid vapour. 



