134 Polarised DaTk-groiind I Ihunination. [Sess. 



consists ill making use of a liigli-angled condenser, or, what 

 does equally as well, a small hemisphere of glass not more 

 than t\ of an inch in diameter, either of which must be 

 brought into contact with the under side of the slip by means 

 of glycerine. To this an oblique pencil of light should be 

 admitted, and a zone of dark ground will extend round the 

 periphery of the illuminator, on the edges of which the diatoms 

 may be seen iridescent, polarised, and resolved to great per- 

 fection. I have found that a green colour on the diatom gives 

 best results — consequently, a blue selenite should be used. 



Another class of subjects equally adapted for examination 

 by this liglit upon a |-incli power are insect scales. Micros- 

 copists know that these objects polarise to a limited extent 

 in the ordinary way, but as the colours are dull this quality 

 gives them no particular interest. It is only when thrown 

 upon a dark ground that their beauty under polar light is 

 fully developed. The scales appear almost as brilliant as the 

 feathers of the humming-bird — microscopically familiar to you 

 all. I consider that this opens up a question for deep optical 

 study, at tlie same time affording one of the most pleasing sub- 

 jects for the microscope ; and as they generally have more than 

 one colour, or rather lustre, the scales are specially beautiful. 

 Amongst those I have examined under this light are Lepisma 

 saccharina, Pieris brassicie, Hepitus humuli, &c. This illum- 

 ination can be applied to low powers by the substitution of 

 the common spot-lens for the stopped condenser — because the 

 emergent rays from most condensers are too oblique, or, in 

 other words, the condenser focus is too short for objectives of 

 long focus. In the case of my Eoss microscope, I have simply 

 to screw the spot into the substage diaphragm-holder, and add 

 the polarising prism underneath as before ; and with this 

 arrangement I can manipulate the light from objectives of 

 from 1-inch to 2 -inch focal length. By this means algce and 

 desmidaceous matter can be splendidly shown. Its remarkable 

 differentiating quality is displayed to great advantage on ana- 

 tomical and other tissues ; for not only are the various parts 

 differently coloured, but the whole has the appearance of relief, 

 as if seen through a binocular arrangement. Micro-crystals 

 also produce fine effects. Some, however, are disappointing — 

 no doubt attributable to the media in which they are preserved. 



