70 



Transactions of the 



rM until ly iM icroscuplcal 

 L Juuriial, Feb. 1, ls70. 



obscure the faint light of the spectra, and entirely prevent the pos- 

 sibility of seeing, let alone measuring, the position of lines or bands 

 in the most refrangible part of the spectrum. 



To produce good effects with this apparatus the upper surface 

 of the compound prism P must make an angle of exactly 45^ with 

 the sides of the tube. Under these chcumstances the limits of 

 correction for the path of the rays in their passage through the 

 dispersing prisms are very limited and must be strictly observed. 

 The nsual method of correcting by the outer surface is inadmissible. 

 For the sake of simphcity, some of the work of the lower part of 

 the micro-spectroscope is omitted in the engraving. As to the 

 method of using this contrivance: with the apparatus just de- 

 scribed measui-e the position of the principal Fraimhofer's lines in 

 the solar spectrum. Let this be done carefully, in hrigrht day- 

 light. A httle time given to this measui'ement will not be thi'own 

 away, as it will not require to be done again. Note down the 

 numbers corresponding to the position of the hnes, and draw a 

 spectrum from a scale of equal parts. About three inches will be 

 found long enough for this spectrum ; but it may be made as much 

 longer as is thought desirable, as the measm'ements will not depend 

 in any way on the distance of these lines apart, but only on the 

 micrometric numbers attached to them. Let this scale be done on 

 cardboard and preserved for reference. Now measure the position 

 of the dark bands in any absorption spectra, taking care for this 

 purpose to use lamphght, as daylight will give, of course, the 

 Fraiinhofer hnes, which will tend to confuse your spectrum. If 

 the few lines occurring in most absorption spectra be now di'awn to 

 the same scale as the solar spectrum, on placing the scales side by 

 side, a glance will show the exact position of the bands in the 

 spectrum relatively to the Fraiinhofer lines, which thus treated 

 form a natural and unchangeable scale (see diagram). But for 



Fig. 2. 



puiT^oses of comparison it will be found sufiScient to compare the 

 two lists of numbers representing the micrometric measures, 

 simply exchanging copies of the scale of Fraiinhofer lines, or the 



