1 30 Polarised Dai'k-groiind Ilhwiination. [Sess. 



or spectral ; and, second, opaque : and, for convenience, I will 

 call the one iridescent and the other opaque. For example, 

 if you examine Aulacodiscus with ordinary dark-ground, you 

 will find the diatom tinted with certain colours for which it 

 appears to have an affinity ; if you now add the polariscope, 

 you will produce an extra colour derived from the selenite. 

 Isthmia enervis, Arachnoidiscus, &c., may be taken as examples 

 of the other or opaque class, because, when viewed by ordi- 

 nary dark-ground light, they appear colourless ; but if the 

 polariscope is applied, they are immediately coated over by the 

 selenite colour, and never present more than one colour at a 

 time. In my former paper, I took the liberty of suggesting 

 possible explanations for the phenomenon of colour in diatoms 

 under ordinary dark-ground illumination, and the subsequent 

 examinations I have made confirm my opinion in these respects 

 — (1), that some diatoms have an affinity, so to speak, for a 

 certain colour, and this colour is evolved from those parts of 

 the frustules only where the markings exist; (2), that it is 

 possible, by racking the spot-lens up or down, to change this 

 colour from one end of the spectrum to the other, demonstrat- 

 ing the great dispersive power of the object ; and, (3), that on 

 account of the form of other diatoms, the pencils of light may 

 impinge upon various parts at angles of greater or less obliq- 

 uity, and thus appear variegated. Bearing in mind, there- 

 fore, that dark-ground illumination is entirely jDroduced by 

 refiection, the foregoing appearances are the direct outcome of 

 the changed angle of incident ray, depending mainly on the 

 focal length of the condenser. 



With regard to these spectral colours, let me explain that 

 Sir David Brewster, in his celebrated experiments upon motlier- 

 of-pearl, discovered by means of the microscope that the sur- 

 face of that material is of grooved or striated structure, 

 resembling that on the tip of a child's finger, and that these 

 grooves are not of uniform interval, but range from 200 to 

 3000 to the inch. It was also discovered that an impression 

 of these lines, taken in wax, or almost any suitably plastic 

 material, gives the same effect, from whence it follows that 

 the beautiful iridescence of this and many other substances 

 emanates. About the same time Sir John Barton succeeded, 

 by cutting fine lines upon steel from 2000 to 10,000 to the 



