VISUAL, GRATING AND GLASS-LENS, SOLAR SPECTRUM. 533 



mere inches of the paper micrometric record, with these decidedly too broad 

 limits of variation : — 



5-90 



5-95 



6.16 



88-24 



89-02 



88-34 



5676 



56-37 



56-27 



Still, however, being facts of observation, unexplained and unexpected, — I 

 have never scrupled to give the anomalies in place thence resulting to any of the 

 spectral lines, exactly as they came out, through all the finished Plates. But 

 in the inferior matter of intensities of lines, and where my method of recording 

 was confessedly weak, — I have often used considerable licence in making each 

 of the three records, if certainly of the same line, approximate from their 

 individual, to their mean, value as to strength. This proceeding will enable 

 every reader to identify the same lines much more easily, in spite of not 

 exactly coinciding places. And though this latter kind of discordance is 

 undoubtedly a blemish, yet its full and intrepid insertion may perhaps prove in 

 the end a valuable aid in deducing the limits of probable error for the place 

 of any line, as given in either a single spectrum representation or in the mean 

 of the three. 



But in matter (b) the immense variations that appear both for place and 

 even existence among the thinnest and faintest orders of Fraunhofer lines are 

 truly surprising and need inquiry before going further. 



Some portions of the uncertainties of place, generally, and for all kinds of 

 lines, thick and thin together, are due to the effects of varying temperature on 

 the grating. Not indeed of the absolute temperature, when settled down 

 to something like permanence ; — for that should have been eliminated by the 

 method of reduction and its appeal to M. Angstrom's standard places. 

 But quick changes of temperature, and sudden springings of the apparatus 

 during a rise or fall of temperature are much more difficult to guard against, 

 and are only too likely to occur from the very nature of the case. That is 

 to say, from the direct heating influence of the Sun, condensed by the heliostat 

 on the slit, and thence passed on to the Grating itself; — but acting there, as 

 dependent on the clouds from minute to minute, sometimes for an hour 

 together, sometimes for only five seconds during several hours. 



Some of the errors of place in special localities of the spectrum were 

 owing to the want of correctly fixed standards of reference in regions where, 

 until lately, nothing was known to exist. And some again are due to the 

 fragmentary character of the observation opportunities afforded by the too 

 frequent clouds, whereby the time interval between noting two successive and 

 neighbouring lines, may have been prolonged from a second, to an hour, or a 

 day, or even a week ; and a spectrum run which should have been con- 



