DESCRIPTION OF THE ACHROMATIC MICROSCOPE. 117 



light wholly polarized in one plane, we have only to- 

 bring one of them under the aperture of the object-glass, 

 to have an admirable polarizer, without being at the 

 trouble of stopping out any of the other pencils. 



"The images, m, n, are much less bright than the 

 principal ones, b, c ; but this is really of no consequence, 

 as we can obtain any degree of light we choose in tiie 

 microscope, either by the condensation of artificial, or 

 the use of solar light. 



Fig 26. 



' 7 ~7 



O O 



"When the vein by which these lateral images are 

 formed is above a certain thickness, their light is white ; 

 but they are most frequently coloured ; and the observer 

 who understands the cause of these colours may make 

 tins coloured pencil of great service in microscopical 

 observations. If he uses a rhomb, which gives to m a 

 green of the second order, it will contain none of the 

 extreme violet and blue rays, and none of the extreme 

 red ; so that it affords a more homogeneous pencil than 

 if it were white light, and thus improves the performance 

 of a microscope that is not achromatic. 



" He may, in like manner, use tints which give the 

 red extremity or the blue extremity of the spectrum, or 

 even, when the tint is divisible by the prism into perio- 

 dical bands, he may absorb the least luminous of these 

 bands, and create a homogeneous pencil of polarized 



