Monthly Mlcroscoplcan 

 Journal, Feb. 1, laTU. J 



Royal Microscopical Society. 



69 



across the centre at right angles to the tube. This hne can perhaps 

 be most neatly produced by photography. I shall be glad to fall 



Ijack on Mr. Davies for assistance in this respect. The lens C, 

 which is focussed by turning the milled ring M, produces an image 

 of the bright line in the field of view by reflexion from the 

 surface of the prism nearest the eye. On turning the micro- 

 meter M, the slide wliich holds the glass plate is made to travel in 

 .grooves, and the fine line is made to traverse the whole lensjth of 

 the spectrum. 



I have described the micrometer as showing a single bright line, 

 because it is thus shown in the engraving ; but in practice it will be 

 found better to employ two bright lines crossed at an angle of 45°, 

 like a letter X. 



It might at first sight appear as if an ordinary spider's web or 

 parallel whe micrometer might be used instead of this contrivance. 

 But on closer attention it will be seen that as the spectrum will not 

 permit of magnification by the use of lenses, the line of such an 

 ordinary micrometer could not be brought to focus and rendered 

 visible. 



The bright line of the new arrangement possesses this great ad- 

 vantage—that it does not illuminate the whole field of view. 



If a dark wire were used, the bright diffused liglit would ahnosfc 



