Nature . mator, as the deviation seldom amounts to more thanhalf a degree for the line under examination. With the above echelon the D lines of sodium areseparated by about sixteen minutes of arc, this beingabout ten times the dispersion of an ordinary good spec-troscope. The successive spectra are only separated byabout two minutes of arc, so that generally two orders ofspectra are visible in the field together ; in practice theechelon can be slightly inclined, thus reducing the in-tensity of one of these almost to zero, so that measure-ments can be made on the remaining one with less riskof
Nature . mator, as the deviation seldom amounts to more thanhalf a degree for the line under examination. With the above echelon the D lines of sodium areseparated by about sixteen minutes of arc, this beingabout ten times the dispersion of an ordinary good spec-troscope. The successive spectra are only separated byabout two minutes of arc, so that generally two orders ofspectra are visible in the field together ; in practice theechelon can be slightly inclined, thus reducing the in-tensity of one of these almost to zero, so that measure-ments can be made on the remaining one with less riskof confusion. One is surprised at the first experience with the instru-ment by the lirightness of the spectra, considering theenormous dispersion and the path traversed by the is no doubt partly due to the incidence on the platesbeing very nearly normal, thus diminishing the lossesby reflection. In making the plates it was a very delicate matter toobtain all of exactly the same thickness. This was. done by first producing a large plane-parallel plate, andthen cutting out the required pieces from it. In the mak-ing of this the ordinary tests for plane parallelismwere found quite inadequate, and every part of it wastested by viewing the interference bands producedbetween the two surfaces while illuminated by a parallelbeam of monoc hromatic light. Each elementary sectionof the surfai e was then refigured until the interferencepatterns were similar throughout. This will give someidea of the patience and skill required to produce asuccessful result. April 27, 1899] NA TURE From the fact of its being necessary to use monochro-matic light, and also from the limited range owing to theshort distance between the spectra, it would appear thatthe use of the new spectroscope will of necessity be some-what confined to special branches of research. For theinvestigation of the Zeeman effect, and the resolution ofhitherto undivided multiple lines, it should prove of greatservice
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