150 Transactions of the 



II. — Note on the Verification of Structure hj the Movements of 

 Compressed Fluids. 



By Dr. Eotston-Pigott, F.E.S., &c. 



{Read hefore the Eoyal Microscopical Society, March 4, 1874.) 



In examining scales, one is often struck by tlie appearance of the 

 obliteration of structure. In many cases tlie structure can be made 

 to reappear by regulated pressure, especially in beaded scales. 



In using Powell and Lealand's dry gtb, of their new and best 

 construction, the covering glass is often found too thick for nice cor- 

 rection ; but in some cases it is just thick enough to permit contact 

 between the front of the objective and the covering glass. In such 

 cases a slight alteration of the focus by means of the screw-collar 

 alone administers a very delicate increase of pressure. 



In this way I have frequently observed the obliterated portion 

 change its form, and this demonstrated the presence of oil natural 

 to the scale, which, under delicate manipulation, may be made to 

 shrink up into a globule, revealing the structure anew, notwith- 

 standing apparent obhteration. 



In these experiments one thing has struck me forcibly — viz. the 

 disappearance and reappearance of minute details. 



In water -lenses, a cracked cover — unfortunately a no uncommon 

 catastrophe — becomes often a most instructive study. For the 

 last five years I have been greatly interested at seeing the sudden 

 discharge of water along grooves, channels, and beads. But of all 

 the liquids used for this purpose of developing structure gradually 

 out of obliteration, I have found none equal to Rangoon oil. 



By this agency of oil and delicately-graduated pressure, by 

 which the covering glass may be made to approach the slide less 

 than the fifty- thousandth of an inch, one, two, thi'ee, then four, then 

 a row, and finally a large surface of beading may be made to peep 

 as it were out of the uniform-looking blank of the oily obliteration. 

 And I would here state, en ])assant, that Rangoon oil seems to 

 have a greater obliterating power than water. 



It is well known now, that when the refractive index of a fluid 

 and of substance are alike, tlie fluid renders the substance almost 

 invisible, except so far as irrationality of dispersion is concerned. 



As an immersion fluid Rangoon oil has some advantages ; but 

 in my hands a solutiun of 1 grain of chloride of gold in 1 drachm 

 of glycerine gives fine efiects. 



In my first paper of 1869 I referred to the ribs of the Podura. 

 It seems probable that the beaded structure is contained within the 

 ribs, and serves to keep the delicate covering membrane distended. 



I am of opinion, too, from recent observations with a oV^h, that 

 the " Podura oil " circulates within this rib and amongst the beads. 



