i8 JV. B. Cartmel 



the left-hand telescope, only that which passes above or be- 

 low the silvered strip reaches the observing telescope, with 

 the result that in the eye-piece may be seen a field illuminated 

 from the left-hand collimator, with the exception of a central strip 

 illuminated from the right-hand collimator; and when the inten- 

 sity of the illumination from both these sources is alike, the field 

 appears uniform. This uniformity may be brought about by 

 varying the width of either of the slits ^ or s'. If the fuchsin film 

 is placed before the slit .s it will cause the center of the field to 

 seem dark, and a match may again be produced by narrowing 

 the slit /, and if we know the original width of the slit, the ratio 



Fig 5. 



F^6 



of the change in width to the original width gives very closely 

 the proportion in which the introduction of the fuchsin has di- 

 minished the intensity of the light. If, instead of diminishing 

 the intensity at s' by narrowing the slit, a revolving sector is used 

 which cuts down the light by the proper amount, the error due to 

 the lack of proportionality between intensity and slit-width does 

 not enter in. However, it is found more convenient to use a 

 sector cut into an arbitrary number of parts, as shown in fig. 5, 

 and as with this we can, in general, only make an approximate 

 match, the varying of the slit-width is used as a fine adjustment. 

 This gives practically as good results as can be obtained with a 

 variable sector. A sector of eighty notches was used except for 



118 



