1887-88.] Polarised Dark-ground Illumination. j 3 1 



inch in producing the well-known buttons, known originally 

 as Barton s buttons." Sir David Brewster, in experimenting; 

 on s eel lines found that grooves of 1000 to the inch, with 

 the light striking at various angles, gave as follows: at 90° 

 white; 74 brilliant blue; 60°, pink; 24°, green; while a 

 specimen ruled to 500 lines to the inch gave yellow through- 

 out. You will thus observe that we have now two factors iu 

 the proposition-namely, (1), fineness or degree of marking: 

 and, (2) angle of incident light. Sir David Brewster ako 

 found, that If the surface of the steel be covered with fluid 

 more orders of colour were developed. For example, it was' 

 ascertained that by applying oil of cassia the result was as 



Colour when dry. 



White. 



Yellowish-rrreen. 



Bhie. 



Yellow. 



Blue. 



Colour when immersed. 



Reddish-yellow. 

 Greenish-blue. 

 Yellow. 

 Bright blue. 

 Yellow. 



I^ow I think you will readily note the significance of these 

 facts, when applied to the behaviour of some of the Diatomacea^ 

 upon the stage of a microscope arranged as I have described 

 in the first place, if the angle of the incident ray is altered 

 by a movement of the mirror, the colour becomes changed- 

 second,_ coarsely lined diatoms, or parts of diatoms, such as 

 ^. balticum, Pmnularia major, &c., the markings of which are 

 from 400 to 800 to the inch, appear yellow or oran^^e irre- 

 spective of change of direction of light or medium in which 

 they are mounted ; and, third, finely marked diatoms, or parts 

 ot diatoms, such as P. angulatum, from 10,000 to the inch 

 upwards in striation, are from pale to bright blue : and, as an 

 example of the effect that a diatom of two different textures 

 presents, I have noticed that Surirella gemma has its main 

 ribs yellow and its fine longitudinal lines blue. Before leav- 

 ing this, I may mention that it is hardly possible to manipu- 

 late so as to be able to compare the change of colour that 

 a single diatom will undergo when dry and mounted in a 



