310 HOW TO SEE WITH THE MICROSCOPE. 



apparent. Mr. Spencer's adjustment of his duplex 

 one-tenth objective (which we were then using) being 

 invariably three divisions of the collar graduations 

 nearer "closed " than my own. 



It is obvious therefore that an " oil immersion " suit- 

 ably corrected to the vision of Mr. Zeiss when worked 

 with ten-inch tube, might require a special correction 

 when in the hands of Mr. Spencer, or myself. Feeling 

 therefore assured of the error embodied in the popular 

 representations concerning the oil immersions of Mr. 

 Zeiss, the author solicited and obtained one of Mr. 

 Zeiss' circulars, which, being official, will, it is believed, 

 be read with interest. With the exception that a few 

 typographical errors have been corrected, the following 

 is otherwise a verbatim copy. ' 



NEW OIL IMMERSION ORTECTIVES. ONE-EIGHTH INCH. 

 ONE-FIFTH INCH. 



" This object-glass is adapted to an immersion fluid > 

 which in refraction and dispersion is equal as nearly as 

 possible to the crcwn glass, a plan proposed by Mr. J. 

 W. Stephenson, of London, as a devise for increasing 

 the aperture and for dispensing with correction. 



" The lens constructed on this plan a four-fold sys- 

 tem of pp. one-eighth focal length, calculated by Prof. 

 Abbe shows an aperture of unusual amount, combin- 

 ing a most perfect definition with a reasonable working 

 distance. It works equally well through thick, thin,, 

 and thick covering glass without needing special cor- 

 rection, owing to the identic refraction of the immer- 

 sion-fluid and the covering-glass. 



