The Astrophysical Observatory 44 J 



There are many other causes of local disturbance, but it 

 should be understood that they are too slight to distort the 

 record, when we are only taking the main features of the 

 solar curve, as is shown in the example just given. But it is 

 when the minuter details of the solar and terrestrial observa- 

 tions are sought that these local disturbances, which are of 

 the same order of magnitude, become especially troublesome. 

 When, therefore, we proceed to make a fuller map of the 

 irregularities of the invisible spectrum than shown above, we 

 are compelled to study the causes of these accidental deflec- 

 tions, and to try to eliminate them, and this necessity has 

 greatly delayed the work, a full account of which will shortly 

 be published. 



The professional reader must be referred to professional 

 papers for an account of the means of overcoming these diffi- 

 culties, but that the general reader may conceive of the re- 

 sults achieved, it is remarked that each inflection of the curve 

 is convertible into a line by a nearly automatic optical process, 

 giving linear spectra, and while the measurements of pre- 

 cision are made upon the original curves, these linear spectra 

 are united by a process of composite photography for the 

 purpose of illustration. That presented on the accompanying 

 plate is obtained by another method. 



With it is given on the scale of mean dispersion the length 

 of the spectrum as known to Sir Isaac Newton " (H A)" 

 to show what the extent of the increase in our knowledge 

 has been. Describing it otherwise upon the scale of the 

 normal spectrum, it may be stated that if the length of the 

 spectrum as observed by Newton be unity, its length as here 

 given would be represented by a little over twelve, and very 

 nearly all of this addition has been made by the application 

 of the processes which have been described. 



A comparison of the three superposed curves with the 

 29 



