1x8 SCIENCE PROGRESS 



covered by a dark red screen, the beam then obtained being visible, in daylight, 

 only to an observer provided with a similar screen. For use at night a screen 

 transmitting only ultra-violet rays could be used in place of the dark-red one, the 

 observer having a screen of fluorescent material in the focal plane of his telescope. 

 The second lamp was devised for naval purposes — to render the ships of a convoy 

 visible to each other at night when no ordinary lights could be shown. The source 

 was a Cooper-Hewitt mercury arc provided with a chimney of a special glass 

 which transmits only one of the mercury lines (X — • 3660 A. U.) and that beyond 

 the limits of the visible spectrum. The light from the lamp could be picked up by 

 means of a special telescope fitted with a barium platino-cyanide screen. In the 

 course of his demonstration Prof. Wood showed some of the very remarkable 

 fluorescent effects which could be obtained by means of this lamp ; it is to be 

 hoped that he will shortly put it on the market in a form suitable for lecture 

 demonstration purposes. 



Bulletin No. 3 of the Department of Scientific and Industrial Research is a 

 paper by L. C. Martin, entitled "A Study of the Performance of Night Glasses." 

 It describes experiments performed to determine the extent to which, under 

 conditions of feeble illumination, the magnifying power of a field-glass or 

 telescope may be increased at the expense of the reduction of the effective 

 diameter of the exit pupil with the consequent reduction of the brightness of the 

 image formed on the retina. The author concludes that for hand-glasses a 

 magnifying power of 5 or 6, with the largest possible field of view, and an exit 

 pupil of 7 or 8 mm. is the best. For stand instruments, provided the object-glass 

 is not less than 3 inches in diameter, the magnification can be increased until 

 the angle subtended by the image at the eye is 1° ; but the magnification 

 required to do this must not be less than 10. It is extremely important, however, 

 if the best results are to be obtained, to screen the eyes from all brighter 

 illumination. Thus, at dusk, when the sky is fairly bright, stray light entering the 

 object-glass and reflected from the walls of the instrument may be fatal to the 

 successful use of the telescope. This difficulty makes it undesirable to increase 

 the magnification above 30 with a 3-in. glass, and, of course, such a large 

 magnification is not suitable for a general survey of the landscape, but merely for 

 the observation of particular small objects. Among the numerous facts brought 

 to light in this investigation is that the sensitiveness of the eye to feeble illumina- 

 tion increases for at least one hour after it is first placed in darkness. 



A joint meeting of the Rontgen and Faraday Societies was held on Tuesday, 

 April 29, for the purpose of discussing the problem of the application of X-rays to 

 the examination of materials opaque to ordinary light. While it is comparatively 

 easy to obtain photographs through considerable thicknesses of substances of low 

 atomic weight (and therefore of small absorptive power) such as aluminium and 

 timber, very considerable difficulties have to be overcome when it is desired to 

 obtain, in this way, information as to the interior character of iron or steel 

 Nevertheless, encouraging results have already been obtained, and M. H. Pilon 

 (the licensee for the Coolidge tube in France) announced that he had succeeded 

 in dealing with thicknesses as great as 55 mm. by giving an exposure of 6,500 sec. 

 with an X-ray tube, taking 6 milliamperes at 120,000 volts. Limits of 40-45 mm. 

 have been reached by several other workers. 1 The industrial importance of this 



1 In an article by F. Janu and M. Reppelen in Stahl und Eisen (which was 

 translated for use at the meeting by Sir Robert Hadfield) it is claimed that flaws 

 have been detected in steel of from 10-12 cm. in thickness; further, Sir Robert 

 Hadfield stated that with new apparatus, now at work in this country, it is hoped 



