80 



KNOWLEDGE. 



[April 2, 1900. 



The glass should, of course, be provided wioh a shutter 

 adapted to varying speeds. At Juvisv, an ordinary 

 Tl'ornton-Pickard shutter, with a maximum speed of 

 1/80 second, has been found very useful. Ifond cameras, 

 supplied with speed regulators, act in an cqti.ally i^atis- 

 factory manner. 



If a cell containing a vcUow solution be used as a 

 screen destined to quench or attenuate the blue of th? 

 sky, the distance separating the glasses need not be 

 smaller than J-inch, or greater than ^-inch. Bichromate 

 of potash, mixed with a few drops of chlorhydric acid, 

 yields beautiful yellows. At the Bureau Central 



Fio. 1.— Fibivd C'in-us, tS!l9, .Vugnst, Id. 2h. JOni., luiul time. 



Meteorologique, M. Angot uses cells ^-inch thick and 

 containing more or less saturated solutions of bichromate, 

 according to the varying intensity of the clouds. The 

 most coloured mi.xture contains 10 per cent, of the 

 yellow substance; another 5, and a third 2i per cent, 

 only. The first screen is advantageous on feeble con- 

 trasts, such as light cirri near the horizon, or in hazy 

 skies. The 5 per cent, solution is generally the most 

 serviceable on ordinary cirri. The last screen is chiefly 

 used on cumuli. Should the contrast between tlv^ 

 silvery crests of the cumuli and the dark blue sky be 

 very marked,, then the coloured screen laight be" dis- 

 pensed with altogether. 



The proper time to give to the exposure is the be- 

 ginner's stumbling block. In fact, the question is of a 

 vei7 complex character, inasmuch as it depends on a 

 large number of factors, such as the angle of the 

 object glass, the diameter of the stop, the sensibility of 



the plates, the saturation of the screen, the luminosity 

 cf the cloud, the sun's altitude, etc. Laying down a rule 

 for the exposure is an impossibility under «uch circum- 

 stances. But the reader may be interested- in the data 

 accompanying the annexed photographs. 



Fig, 2.— CiiTus with Wisps, 1899, August, Id. 2h. 3.5ra., local time. 



Plate. Fig. 1. — Cirro-cumuli trausitintfl across the Sun ; 

 following and preceding wet weather. Photograplijtaken 

 with an object glass 10-1 in. aperture and 312 in. focal 

 length. No yellow screen. Stop=^^ . Exposure = j'j 

 seeoud. 



Fio. 3.— Cloud Ripples, lHt»9, September, 25(1. 2li. 23iii.. loe.il time. 



Plate, Fig. 2.— -Gigantic thunderstorm Cumuli during 

 hot, showery weather. Same object glass. Slight yellow 

 screen. S:top=j.C. Exposure=A second. 



Fig. 1 (text). — Cirrus in biinds, attending a barometric 

 fall after fine weather. Same oliject glass. Strong vellow 

 screen. Stop = J-. Exposure=l second ; lengthened on 

 account of the yellow's absorption. 



