MOUNT WIT>SON SOLAR OBSERVATORY. 229 



Perhaps the point of greatest interest in connection with the instru- 

 ment is the possibiHty it affords of eUminating personal errors from 

 the measurement of the positions of spectrum Hues. Owing to the 

 fact that the existing instrument was constructed for photometric 

 uses rather than for the precise measurement of positions, it has 

 not been possible to test this feature fully. This much appears 

 certain, however: Under favorable conditions the maximum of the 

 registered curve representing a spectrum line may be determined 

 with a precision corresponding to a few tenths of a micron on the 

 original negative. Among the lines actually tested are several of 

 those observed for the determination of the sun's general magnetic 

 field, whose width and difTuseness render measurement by the usual 

 method extremely difficult. Favorable results were also obtained 

 from lines of stellar spectra too diffuse for measurement by the 

 ordinary methods. In fact, wide lines give results superior to those 

 from lines narrow and sharp, for a wider slit can be used in front of 

 the photo-electric cell, with the result that irregularities in the curve 

 caused by abnormal distribution of silver grains are greatly reduced. 

 The same end may naturally be accomplished by lengthening the 

 slit, but with narrow stellar spectra this can be done only to a limited 

 extent. The results indicate that the only obstacle in the way 

 of a substantial gain in the precision of spectrum measurements is 

 the construction of a suitable mechanism for driving the microscope 

 stage and the photographic plate at uniform speeds. An ultimate 

 precision of one micron seems easily possible. 



Although the test of the instrument for the measurement of the 

 positions of spectrum fines is incomplete, it has been possible to inves- 

 tigate with some detail its applicability to the study of intensity 

 distribution. It has been extensively used for this purpose in con- 

 nection with lines in various classes of spectra, including sun-spot and 

 normal solar spectra, spectra of Arciurus and Nova Geminorum No. 2, 

 of the tube-arc, and of other terrestrial sources under varying condi- 

 tions of pressure, magnetic field, etc. The results for lines showing 

 complicated Zeeman structure are very interesting and suggestive. 

 The fidelity with which the apparatus reproduces minute variations 

 in photographic density as shown by successive registrations of the 

 same line or spectrum is remarkable. A highly important feature is 

 the revelation it alTords of the part played by physiological error in 

 the ordinary examination of spectra — lines apparently separated are 

 shown, for example, actually to be blends, the separation being a 

 contrast effect. 



Finally, preliminary experiments have been made in the direction 

 of adapting the instrument to the measurement of the total amount 

 of silver deposited in star images, with the idea of applying it to 

 stellar photometric investigations. The advantages of such a method 



