446 H. von Mohl on the Arrangement of the 



Many opticians place the analyzing prism immediately behind 

 the object-glass. This certainly has the advantage of allowing 

 the eye to be placed close to the eye-piece, so that a view of the 

 entire field can be obtained. Brit this unimportant advantage is 

 counterbalanced by decided objections, and especially by the 

 circumstance that the sharpness of the image is inoi'e or less 

 impaired when the light between the object-glass and the eye- 

 piece is allowed to pass through such a massive body as a Nicol's 

 prism, the surfaces of which, moreover, are not usually ground 

 perfectly flat. It is therefore indisputably better to place the 

 prism above the eye-piece, where it much less impairs the beauty 

 of the image. The objection that the held is thus limited is of 

 no importance, as it is easy to find a prism with which this de- 

 fect does not occur, or merely at the outer margin, which is of 

 little consequence. If the prism is fixed in a tube adapted to 

 the eye-piece, we have, moreover, the advantage that it can be 

 rotated on its axis at pleasure, and may be easily removed and 

 replaced; whilst placing the prism in the tube of the microscope 

 and rotating it within the latter is a complicated arrangement, 

 and involves a waste of time. 



Passing to the microscopic apparatus : in the case of this also 

 the greatest possible intensity of light is the first condition of 

 successful results. The most important point is the illuminating 

 apparatus. As regards the mirror, an ordinary plane one may 

 be used ; but the best consists of a not too small glass prism, 

 the section of which forms an equilateral triangle. If the sur- 

 faces of this are from 15 to 18 lines in length, that is sufficient. 



It is essentially necessary that the parallel rays of light, after 

 traversing the Nicol's prism, should be strongly condensed upon 

 the objects. The best apparatus for this is an achromatic con- 

 denser consisting of three lenses of about 3 lines focus and large 

 angle of aperture, such as exists in the large English microscopes. 

 Stops are only injurious for the present purpose ; but it is evi- 

 dent that the condenser must be capable of being placed in the 

 direction of the axis of the microscope, so that its focus may 

 fall exactly upon the objects. Amici, whose advice I solicited, 

 in case he might improve my arrangements, wrote to me in the 

 negative, considering my apparatus as satisfactory, but suggested 

 that I might substitute a colourless flint-glass plano-convex lens, 

 5 lines in diameter, and with the convex side downwards, for the 

 achromatic condenser. The result was perfectly satisfactory, espe- 

 cially when ordinary day-light was used, and by no means inferior 

 to that obtained with various achromatic condensers ; but when 

 sun-light was used, as described below, this lens was not so good 

 as the achromatic condenser. For the polarizing microscope to 

 yield a good image, the concentration of the light upon the 



