384 
ME. J. EVERSIIED ON WAVE-LENGTH DETERMINATIONS, ETC., 
The bright radiations of the chromosphere are in no cases strong enough to .show 
as hriglit lines upon the continuous spectrum, although they are strongly impressed 
along the borders, where they form in many cases a continuation of the dark arcs of 
the Fraunhofer spectrum. The very strong Ijright arcs, H and K, can be traced 
round the limb of the moon for a greater distance than any others. 
No. 2 spectrum appears to have been under-exposed and the bright arcs are rather 
difficult to detect upon it, although the exposure was made within 10 seconds of 
second contact. 
Measures were not made of the bright lines in either spectrum as they are not verv 
well adapted for accurate determinations. All of them can be identified with the 
stronger lines in No. 3 spectrum. 
Methods of Measurement and Beduetion of the Spectra. 
The Fraunhofer spectrum was carefully measured in both cusp spectra and identified 
line by line with Higgs’s photograjihic map of the normal solar spectrum. 
As these measures form the basis for the determination of wave-lengths in spectra 
No. 3 (flash) and No. 4, it is desirable to give a brief account of the methods adopted. 
I am indebted to Dr. Rambaut for the loan of an excellent millimetre micrometer 
belonging to the Radcliffe Observatory. 
This instrument consists of a microscope mounted on a sliding frame, and moved by 
a long screw with a pitch of 1 millim. The head of the screw is divided into 
100 parts, and by estimation can be read to ’001 millim. All the measures were 
made upon this micrometer. 
Fach spectrum was measured completely three times over, using different sections 
of the screw and different readings of the head each time. By this means any 
systematic differences due to errors in the screw were eliminated in the mean result, 
and accidental mistakes in reading the divisions were readily found and corrected. 
No evidence of any systematic error in the screw was, however, detected, and the 
three sets of measures of spectra Nos. 1 and 2 showed a very satisfactory accordance 
throughout the whole length measured. 
In measuring a line, the mean of three settings of the spider lines was taken as the 
scale reading of the line, and in the final mean result, in which the two cusp spectra 
are combined, every line represents eighteen settings of the spider lines. 
Tlie measures of the two cus]) spectra when compared were found to agree as 
closely as two separate measures of either spectrum, thus proving that no change had 
occurred in the disjiersion from any cause. The two spectra were therefore considered 
to be identical, and the measures were confliined. 
From the final mean of the six sets of measures al)out fhirfy Avell-defined and 
isolated lines were selected as standards ; these are all distributed flu-ongh flie ultra¬ 
violet jiortion of the spectrum more refrangilfle than H. In the region betAveen 
