ZOOLOGY AND BOTANY, MICROSCOPY, ETC. 93 



iliuminant by the substage condenser. The instrument consists of a 

 heavy stand C with an upright rod I), up and down which the lamp can 

 be moved with a rack-and-piniou E. The glass rod A is fixed to a 

 block F, which can be pivoted at an angle so that the light may be 

 directed either up or down, rendering the lamp useful for every kind of 

 illumination for either high or low powers. The block F carries a 

 bar G which carries the fitting H of the electric light B, and also the 

 fitting J of the bull's-eye K. The bar G can be moved and clamped in 

 the block F, thus giving a rough adjustment for moving the lamp B, or 

 the bull's-eye K, nearer to or further from the glass rod A. The 

 lamp B and the bull's-eye K can also be both moved by rack-and-piniou 

 motions along the bar G. The lamp B when placed close to the end of 

 the glass rod A gives a very intense illumination, as a large proportion 

 of light enters the glass rod ; but as it is moved away by the raok-and- 

 pinion the intensity of the light is reduced rapidly, varying according 

 to the square of the distance of the lamp B from the end of the glass 

 rod A. The condenser K can be swung out of the way when not re- 

 quired, and can be focused by means of the rack-and-pinion so as to 

 give parallel light or to focus the light to a small area for the illumina- 

 tion of opaque objects. In front of the illuminated disk end of the 

 glass rod an iris-diaphragm moved by a lever L is placed, which enables 

 the illuminated disk to be reduced in size, and which forms when closed 

 down the best object by which to focus the disk upon the microscopic 

 object with the substage condenser for producing critical illumination. 

 In front of the iris-diaphragm is a stage M which carries a trough N for 

 acetate of copper, or other monochromatic solution, and by means of 

 clips, glass colour filters, patch stops, or other appliances, can be attached 

 to this stage. 



i &> v 



New Form of Polarimeter for the Measurement of the Refractive 

 Index of Opaque Bodies.* — W. T. Barrett's instrument for the above 

 purpose depends upon Brewster's well-known principle that the index of re- 

 fraction of any substance is the tangent of the angle of maximum polari- 

 sation for that substance, and that, hence, when a ray of light incident 

 on a transparent body is polarised by reflection, the refracted ray forms 

 a right angle with the reflected ray. By means of Brewster's Law the 

 indices of refraction of various opaque non-metallic reflecting surfaces 

 have been obtained. As every different colour has a different index of 

 refraction, the law shows that the polarising angle correspondingly varies 

 with the different rays of the spectrum, being, for a given substance, 

 smallest in the red and largest in the violet. In bodies of low dispersive 

 power the angle of maximum polarisation is nearly the same for all 

 colours, and white light can be used as the source. In other cases mono- 

 chromatic light must be employed — either a sodium flame or suitably 

 coloured glass in front of the source described below. The amount of 

 light reflected from some opaque bodies is small, and hence the deter- 

 mination of the polarising angle is difficult, unless we can always keep 

 the analyser placed in the reflected beam at the same angle as the ray 

 incident on the opaque surface under examination. To secure this, the 



* Sci. Proc. Roy. Dublin Soc, xii. (1909) pp. 98-901 (2 figs.). 



