[J.O.S.A. & R.S.I., 7

There is therefore no precision connection between back focus andtube lengths, but it is customary to correct for the position determinedby the tube length and objective mounting.

In the microscope interferometer the negative "tube length" lens ismounted so that the distance from the objective shoulder to the backfocus is from about 140 mm to 180 mm; or if a 250 mm tube length

lens be supplied, an adjustment from about 230 mm to 270 mm isprovided.

RESEARCH DEPARTMENT,ADAm HILGER,

LONDON, ENGLAND.

BIBLIOGRAPHY

HILGER INTERFEROMETERS

1. Patent No. 103832/16, and corresponding foreign patents (covering the prism andlens interferometer of which one form is described in ref. 3).

2. Patent No. 130224/18, and corresponding foreign patents (covering the cameralens interferometer of which one model is described in ref. 6).

3. "Interferometers for the Experimental Study of Optical Systems from the point ofview of the Wave Theory," F. Twyman, Phil. Mag., 35, January, 1918, p. 49.

4. "On the Use of the Interferometer for Testing Optical Systems," F. Twyman.The Twenty-First Annual Traill-Taylor Memorial Lecture. Delivered before The Royal

Photographic Society of Great Britain, October 15, 1918.5. "Correction of Optical Surfaces," F. Twyman, Astrophysical Journal, 48, No. 4,

November 1918.6. "An Interferometer for Testing Camera Lenses," F. Twyman, Trans. of Optical

Society, 22, No. 4, 1920-21.7. "The Testing of Microscope Objectives and Microscopes by Interferometry,"

F. Twyman, Trans. Faraday Soc., 16, Part I., September 1920.8. "An Interferometer for Testing Camera Lenses," F. Twyman, Phil. Mag., 42,

November, 1921 (reprint of reference 6).9. "Fringe Systems in Uncompensated Interferometers," J. Guild, Proc. Phys. Soc., 33,

40.

Spectrum of the Aurora Borealis.-In a photographic study of thespectrum of the aurora between wave lengths 3135 and 6465 a largenumber of lines and bands of nitrogen were observed, no other knownline, and four otherwise unknown lines, (one of them the celebratedgreen line). Since the relative brightnesses of the green line and thenitrogen lines are the same at different levels of the aurora, Vegardconcludes that all the unknown lines are also lines of nitrogen, perhapsresulting from ionizations of higher degree than any observed in thelaboratory. [L. Vegard, C.R. 176, pp. 947-950; 1923.]

K. K. DARROW

F. TWYMAN656