Royal Microsco]^ical Society. 151 



For I Lave noticed a movement of fluid in the direction of the 

 spurious spines, under the circumstances previously described. 



In these experiments of dehcate differential pressure, varying 

 from the sixty-thousandth to the ten-thousandth of an inch, the 

 portion first exposed to view by the recession of the oil is one of 

 the most tangible tests of reality of structure it is possible to con- 

 ceive. A rib shoots out as a rib, and a minute bead peeps forth 

 by degrees — not jjer salium. Bead joins bead ; a short string or a 

 small cross or chain of beads appears and disappears as the oil 

 recedes like a wave, and again overflows the structure : exactly as 

 the pressure is delicately increased or diminished the same par- 

 ticular bead or beads can be thus brought up from invisibility to 

 a rare prominence and distinctness, adorned with colour and shadow 

 if oblique light is used. But direct light from a plane mirror 

 (without a condenser) reflecting the light from a white blind in 

 sunshine answers extremely well. Otherwise a l-J-inch Ross ob- 

 jective forms an excellent illuminator from direct light without 

 reflexion from a mirror. 



A Tolles' -^th with a lengthened tube and eye-piece shows them 

 very well, and in justice to Mr. Tolles I must say that it is a true 

 |th, magnifying exactly 600 times under the same circumstances 

 that the Powell and Lealand " eighth " magnifies 800 diameters. I 

 am indebted to Mr. Frank Crisp (Memb. Council Roy. Mic. Soc.) 

 for the loan of this interesting glass, marked 98° balsam angle. 



I see the beads very well also with a very fine Wray ith. 



This method of investigation, of course, as only showing the 

 parts that are left high and dry, disposes, once for aU, of the 

 question of spurious beading, fondly ascribed to optical illusion. 



It may not be uninteresting to mention that in focussing down- 

 wards with the dry Powell and Lealand ^th. At first : — Shaded with 

 a blue diaphragm, the beads appear dark red, some lighter red, 

 and others approaching white. Deeper : — The shades all pass into 

 white. Deeper still : — Dark blue appears the predominant colour 

 of the beading. Then, lower still, these colours recur over again, 

 apparently from the effect of a double layer lying beneath and 

 betwixt the upper. 



Careful examination shows the test-oil clings to the under sur- 

 face, but gradually oozes upwards by pressure, and then by degrees, 

 as it were, completely dissolves out the beads by obliteration. It 

 forms small globules, giving minute diffraction rays and a brilliant 

 focal point, according to the degree of pressure, or what is the same 

 thing, optically, the degree of space between the upper cover and 

 the lower slide.* 



* The splendour of the definition was demonstrated by the jet blackness of 

 cylindrical forms. 



At this stage of more perfect microscopical definition it is not without piquancy 



