ZOOLOGV AND BOTANY, MIGUOSCOPV, KTC. 397 



to form an imuge on a sensitive plate held in an ordinary camera. For 

 visual observation, the plate-holder was removed, and the image viewed 

 with an eye-piece of 3 c.cm. focal length. This moderate magnification 

 was found to be of considerable advantage, though of course it does 

 not increase the intrinsic brightness of the scattered light. 



With ordinary untreated air in the apparatus, a very bright track 

 due to scattering by the dust particles was observed. Passing a current 

 of air dried by phosphorus pentoxide, and filtered by cotton-wool, the 

 dust particles became fewer. It took a little time to get rid of them, 

 because the shape of the vessel was unfavourable for a wash through of 

 air. After a few minutes, however, the dust particles became so scarce 

 that they could only l)e seen occasionally crossing the track, and they 

 soon vanished entirely. 



At this stage there was a blue track along the beam, which, though 

 much fainter than the original dusty track, was visible without difficulty 

 when the eyes had been rested in the dark. 



When the vessel was exhausted, the blue track disappeared, nothing 

 remaining except the above-mentioned ring of light diffused by the 

 vessel. Readmitting filtered air, the blue track reappeared exactly as 

 before. 



These various tests and precautions seem to prove beyond doubt 

 that the observed scattering was not due to dust surviving imperfect 

 filtration, or to any false light, but that it is properly of pure air. Its 

 intensity is of the order of magnitude to be expected. 1. By proper 

 arrangement of the experimental conditions it is possible to observe the 

 scattering of light of pure air, free of dust, in a small-scale laboratory 

 experiment. 2. Similar results can be obtained with other gases. 

 Hydrogen gives much less scattering than air, oxygen about the same, 

 carbon dioxide decidedly more. 3. The scattered light in air and in all 

 the other gases is blue — the blue of the sky — illustrating very directly 

 the theory that attributes the blue of the sky to scattering by the mole- 

 cules of air. 4. The scattered light is almost completely polarized. 



J. E. B. 



Some Principles of Illumination.— M. A. Ainslie {Proc. Photo- 

 micrographifi Soc, 1918, 7, 1-23, 6 text-figs.) analyses the elements 

 of the subject clearly and concisely. C S. 



The Polarization of Lig-ht.— F. C. Reynolds (Proc. Photomicro- 

 graphic Soc, 1918, 7, 24-30, 4 text-figs.) gives an elementary description 

 of the principles involved. C. S. 



The Barnard Incandescent Gas Lamp, manufactured by H. F. 

 Angus and Co., may be used either with an ordinary Bunsen flame (4) or 

 witii a high-pressure methylated spirit-burner. It embodies a somewhat 

 new but very simple method. It consists of a specially made incan- 

 descent mantle in a metal tube (1), which gives an intense uniform 

 source of light of about 5 mm. across. It is sufficiently intense for 

 seeing Spironema pallidum, and for differentiating it from other similar 

 organisms. In front of the light is placed a bull's-eye or condensing 



