Polarising Microscope in the Examination of Organic Bodies. 1 17 



object must be so great, that delicate transparent objects, 

 such as anatomical vegetable preparations, must be almost in- 

 visible in the bright light of the held before the analyser is 

 applied ; an amount of illumination such as is used in ordinary 

 microscopic observations is totally insufficient ; and this is the 

 principal reason why such imperfect results have been obtained 

 by most observers with the polarizing microscope. 



"With regard to the object-glass, those who have several at 

 hand will do best by using such as transmit most light. For 

 low powers, magnifying about 200 diameters, the better kind of 

 German object-glasses, e.g. the Nos. 1 and 2 of Keller's micro- 

 scope, will answer every purpose ; but where higher powers are 

 required, as the gth, |th, and T ',jth, the English glasses are very 

 much the best, especially those made by Ross. The highest object- 

 glasses will of course be used only for the most delicate objects, 

 in which minute details are to be investigated ; but, when the 

 illumination is well managed, they transmit sufficient light to 

 allow of the most difficult object being seen distinctly. 



Equally important with the arrangement of the polarizing 

 microscope, and the selection of the pieces of apparatus ensuring 

 the greatest intensity of the light, is the choice of the source of 

 the light used for illumination. 



When the power required does not exceed 200 or 300 dia- 

 meters, and when the object exerts considerable polarizing power 

 (as, e.g., the membrane of most vegetable cells, the fibres of 

 spiral vessels, the granules of starch, &c), common daylight is 

 sufficient ; and on many rather dark days in the winter I found 

 no difficulty in making these investigations. A far more favour- 

 able result is certainly obtained, on those days when there is full 

 sunshine, by inclining the mirror towards a part of the sky not 

 far from the sun. The light from white cumuli also affords 

 good illumination, except that the constant change of the bright- 

 ness interrupts observation and fatigues the eyes. These sources 

 of illumination, however, are insufficient in the case of very dif- 

 ficult objects, in which either a very high power is required, 

 or which act but feebly upon the polarized light ; thus, to give 

 an instance in a well-known object, on illumination by ordinary 

 daylight I could distinctly recognize the double refraction pro- 

 duced by the silicious carapace of Pleurosigma angulatum, but 

 I could not detect the six-sided dots in it. In such cases it 

 requires a far more intense illumination to obtain a bright and 

 sharp image. In regard to this point, all may be accomplished 

 by adopting the arrangement of the solar microscope, the direct 

 light of the sun being received upon a mirror and allowed to fall 

 perpendicularly upon a ground-glass plate fixed in the shutter, 

 the condenser being so arranged that the image of this illuini- 



