1869.] EDWARDS — ON MICROSCOPIC ACCESSORIES. 275 



ditto " for the eye- piece. Accuracy in the expression of the size 

 of objects described is often of the greatest importance in original 

 investigation. 



Must T call the "Brook's nose-piece" an accessory? I sup- 

 pose I must ; yet I am accustomed to regard it as one of the 

 most essential parts of my instrument. So valuable is the ready 

 exchange of two object glasses, that it is almost essential where 

 high powers are often used. I may here state incidentally that 

 I have tried the multiplication of this rotatory movement to the 

 extent of three and five object glasses, but have found that the 

 strain and leverage upon the fine adjustment is too great, and 

 therefore prefer the double nose-piece. 



The polariscope is an essential part of a good microscope. 

 It cannot be dispensed with, and should therefore not be deemed 

 a mere accessory. Still it often is so called, and various con- 

 trivances have been suggested to reduce its cost. I should 

 recommend, however, large prisms and thin selenites, and would 

 by no means advise any experiments with tourmalines or Hera- 

 pathites. I have tried them, and find no polariscope is so satis- 

 factory as the Nichol's prism, with a good selenite below the 

 object ; the analyser being placed immediately behind the object 

 glass. In the Harley Collins' microscope, the arrangement is 

 excellent ; the same slide carries the analyser and the binocular 

 prism into the body, thus saving a great amount of screwing and 

 unscrewing. 



I would urge upon the microscopist the importance of this 

 mode of illumination, as it frequently gives valuable delineations 

 when least expected. Objects required for investigation, such as 

 parasites and .tissues, are often obscure and unsatisfactory by 

 ordinary modes of illumination, owing to their great transparency. 

 In these cases polarized light is of the greatest assistance in the 

 delineation of structure. 



Some confusion exits in the popular mind as to the compara- 

 tive value of the polariscope and a mode of illumination intro- 

 duced by Mr. Heys, of Machester, of a very different character, 

 called the kalescope. By passing the rays of light through the 

 bull's-eye, obscured by a coloured glass, and through the con- 

 denser below the stage, through a diaphragm of various coloured 

 glasses, Mr. Heys obtained brilliant fields. Opaque objects are 

 thus illuminated with bright lights of difierent colours, and are 

 tinted with fringes of considerable beauty and delicacy. But 



