few of the principal lines ; at least not until a comparison of the results with those 

 of Angstrom and othei-s should indicite that it was desirable. Angstrom based 

 nearly all of his work upon uiicrometric nu^asnrements from these principal lines 

 and if it can he shown that they are at least rclntli-ehj correct there is strong 

 presumptive evidence in favor of his niicionictric incisures. Concerning my 

 instrumental appliances, a comparison with those used by Augstrüm (I have been 

 unable to tind descriptions of the spectrometers used by the other observers 

 referred to ) will show that in some resjiects, at least, they are decidedly superior. 

 In dimensions and fineness the grating far exceeds his, giving ranch greater 

 dispersion and, iloubtless, su[)orior definition and the angular measurements 

 were capable of being male with a greater degr>'o of precision. Tn .short, even 

 if the value of the fjratiiKj sprtce was unkiioa-ii, I ought, with reasonably good 

 observation work, to be able to decide in fivor of one or the other of these 

 discordant sets of results, or, possibly, against them all. 



The work was done during the months of November and December ou 

 account of the unusually clear weather which nearly always prevails here at that 

 season; the prevailing low temperature at that time of the ye.ir i.^, however, an 

 objection. 



THK SPECTllOMETKH. 



e 



The general construction and relative dimensions of tlu; various [urts of th 

 instrument are shown in the accompanying photograph. The instrument was 

 made in the winter of 1879-80 by Messrs. Fauth and Co. of Washington D. C, 

 Ü. S. A. The mechanical execution of the work was excellent in every respect 

 and higldy creditjvble to the makers, whose reputation for the production of 

 instruments of precision is well established. 



For many valualjle suggestions concerning the details of its construction 

 and for a carefid examination fif the instrument after its completion and before 

 it was sent to Japan, I am indebted to Professer C. A. Young of Princeton 

 College, and I wish to acknowledge witli gratitude this, among the many obliga- 

 tions under wliich he has placeil me and. indeed, every stutlent of spectm.scopy. 



Tlie diiriensions of the instrument are nearly as follows; — 



Diameter of Circle 30 cm. 



Ijongth of View & CoUimating Tclcscopee 35 cm. 



A|KTtiire ,, ,, „ „ ;") cm. 



Height,— to circle 32 cm, 



Tlic circle is divided to spaces of r>' each. It is read by two microscopes 

 diametrically opposetl U) ench other, tlio eye pieces of which an; jirovided with 

 inicrometerH, one turn of tlie screw being erpial to 1'. Thn micronjcter screw 

 lieads are divide<l into sixty part.s. thus carrying the reailin;; to single seconds. 

 The varif)us part« are cjii»ahie of almost every dcsiralil" movement relative to each 

 other. The lower arm., supporting the cojiimatiiig telescope, swings completely 



