174 Phenomena of the Podura Test. 



lution would be primarily cylindrical, secondarily molecular : as a 

 third and higher order of excellence the bright band of the secondary 

 spectrum would diminish in breadth, and each spherule would pre- 

 sent under direct illumination a focal point of light. 



Mr. Slack's Silica Films. — No more instructive object has been 

 seen than these shdes. First of all, we know what we are looking 

 at and what we ought to see. When our Secretary, with the 

 humour he so richly possesses, suddenly asked me at the Society's 

 room what I saw on the microscopic stage, through an old Koss' ^, 

 I at once said, " Some cylindrical fibre, but what, I could not tell ! " 

 He then informed me they luere cracks in a film of silica ; yet they 

 seemed to me absolutely cylindrical fibres. 1 have noticed the same 

 appearance in Nobert's lines. Here we see what the microscope 

 may misrepresent. The interval between the edges of the two 

 separated films was filled up by a phantom-cylinder ! Two square 

 edges were thus transfigured. It is evident that there ought to have 

 appeared two Hack lines, with more or less prismatic colour re- 

 Buitiug from the usual refractions at right-angled prisms. This 

 experiment is exactly the reverse of the former. There were no 

 headings, but cylinders conjured from square glass or silica edges ! 



"When the definition of these edges was accomplished in the 

 most perfect manner obtainable, two black edges with a secondary 

 spectrum were distinctly observed with Powell and Lealand's best 

 immersion §^th and a variety of other glasses. 



There is one other cause of colour observable. Let us suppose 

 we have resolved the scale Podura macrotoma into ribs, and then the 

 ribs into parallel rows of bluish and reddish beads : now rotate the 

 scale slowly, and these colom's disappear. Some are of opinion that 

 these colours are due to diffraction, others to polarization of thin 

 films. However that be, these double beads are generally in- 

 visible in perfection, except when showing nearly complementary 

 colours.* 



Molecidar Ribbing and their Intersections. 



Advancing now in the scale of difficulty and double intersecting 

 rouleaus of beads crossing at various angles in contiguous planes, 

 above and below, we get we will suppose : — • 



Primary Definition. — At the exact places where these rouleaus 

 cross there is at first sight an obliteration. Where two spherules 

 lie exactly the one over the other, a point of bright hght is developed. 

 In this way the Podura scale may be made to appear studded over 



* Our late highly-esteemed President brought me this specimen to resolve 

 for him; the charming delight which he displayed on seeing their complete 

 resolution with a half-inch objective made a lasting impression of his unaflected 

 amiability and candour. I subsequently had the honour of showing them to 

 Dr. Lawson with an inch objective. 



