A Polariscope for Dissecting and Pocket Microscopes. 151 



directions so that those rays incident on one at the least an^fle 

 would fall on the other at the greatest angle of incidence, but 

 unfortunately an analyser so constructed would be of too great a 

 length to be used except it be placed in the tube of a compound 

 microscope between the object glass and the eye-piece. It would 

 not be suitable for the simple microscope. 



To fix the cover-glasses inside a blackened tube at the required 

 obliquity is not such a simple operation as it might seem, especially 

 as it is important to leave no gap through which light can escape 

 between the cover-glasses and the sides of the tube. The plan I 

 find best is to use a tube of black paper instead of cardboard, and 

 to have the top and bottom covers hinged at their edges to slips of 

 black paper which may be drawn through the tube until the pile 

 is adjusted to the proper angle. The free ends of the slips may 

 then be turned down outside, leaving however a very little free 

 play, and the whole enclosed in an additional outer tube which may 

 be of cardboard. This arrangement enables the cover-glasses to be 

 taken out and cleaned from time to time, and it also allows of the 

 angle of incidence being adjusted, being increased by slightly 

 compressing the tube laterally and decreased by drawing the pktes 

 together by means of the slips of paper. 



The attachment of the outside covers to the black slips does 

 not really reduce the field of view, but has rather the effect of 

 cutting oif the light reflected from the edges of the cover-glasses, 

 which would otherwise be trouldesome. The fittings required to 

 attach the tube to a pocket lens or dissecting microscope can be 

 constructed without much difficulty. 



For -use with my compound microscope I have fitted an old 

 eye-piece with a pile of cover-glasses between its two lenses. It 

 gives a somewhat limited field of view, but it has the advantage of 

 obviating the seven operations of screwing and unscrewing required 

 to attach or remove the Nicol prism analyser, and substituting 

 the simpler operation of changing the eye-piece. I have also 

 aiTanged a polarizing frame to slip over the mirror, giving a better 

 illumination than is obtainable with the Mcol. "With careful 

 adjustment this home-made polariscope shows the crosses on small 

 starch grains much better than one w^ould have expected under the 

 circumstances. 



An important point to be borne in view is that only a very 

 small portion of tlie incident light reaches the eye through the 

 polariscope, and it is therefore very essential, especially in working 

 with a dissecting microscope, to screen off any light which would 

 otherwise fall on the object from above. Otherwise the object 

 will be visible as an ordinary opaque object on a dark background, 

 and the polariscopic effects will be unnoticeable. In cases of doubt 

 a selenite may be used with advantage, as doubly refracting objects 

 will show colours when the analyser (not the polariscope) has 



