of Iodine and Bromine Vapour by other Gases. 



311 



and distilled water, avoiding the use of alcohol and ether for 

 drying. A few small crystals of iodine are now introduced 

 into the flask, the neck drawn down to a 1 mm. capillary at 

 one point, and the flask thoroughly exhausted with a Gaede 

 or other mercurial pump. It is most important to have a 

 very perfect vacuum (less than '01 mm.), and it is usually 

 necessary to keep the pump running for 15 or 20 minutes 

 to secure this, if the capillary is narrow. The flask is now 

 sealed, and can be used for demonstration purposes at any 

 time. It requires no heating, for the iodine fluorescence is 

 brightest at the pressure which the vapour has at room 

 temperature. We have only to hold the bulb in the con- 

 verging beam furnished by the condensers of a large pro- 

 jecting lantern, or the condensed beam from a heliostat. If 

 a lantern is used it is best to throw the beam upwards, as the 

 reflexions give less trouble. The intensely brilliant cone of 

 yellowish-green fluorescent light can be seen from the back 

 of the largest lecture hall. The aberration of the spherical 

 lenses is well shown as well. 



The bulbs used in the photometric work were smaller, 

 having diameters of about 7 cm. 



The fluorescence was excited by sunlight reflected into a 

 dark room by a large heliostat, and concentrated to a focus 

 at the centre of the bulb by a Voigtlander portrait objective 

 of 12 cm. aperture and 27 cm. focus (F. 2*3). The work 

 was done only on very clear days when the sky was free 

 from haze and clouds. A Welsbach light was used as a 

 standard, its colour being brought to a match with that of 



Fig. 1. 



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the iodine fluorescence by a combination of pale cobalt glass 

 and a dilute solution of bichromate of potash. The arrange- 

 ment of the apparatus is shown in fig. 1. Between the eye and 

 the brightest part of the fluorescent cone is mounted a small 

 about 1x3 mm., made by silvering a 



