320 



OPTICAL PROJECTION 



solution, but also a red fluorescence, though rather more dull. 

 Cyanosine (methyl-tetraiodo-fluorescein) Mr. H. G. Madan 

 tells me gives a fine orange fluorescence. It too must be 

 dissolved in alcohol. And Mr. Jewsbury tells me that fifteen 

 parts glacial acetic acid and one part essential oil of pepper- 

 mint, heated to nearly boiling point, give a red fluorescence. 

 Saffron in alcohol fluoresces red-brown. 



Of blues, the sodium salt of B-naphtholsulphonic acid 

 fluoresces rather more powerfully than quinine. Nearly all 

 the petroleum lubricating oils fluoresce blue more or less, and 

 samples can be found that do so rather powerfully with the 

 lime -light. Such give the strongest blue I know as yet, but 

 not sufficient for designs on paper. The aniido compound of 

 phthalic acid fluoresces a bluish-green, but best in electric or 

 magnesium light. 



182. Phosphorescence. This phenomenon is readily 

 shown by exposing a large sheet of card coated with 

 Balmain's luminous paint to the beams 

 of an electric lantern or burning 

 magnesium, interposing something to 

 cast a distinct profile or shadow. 

 After some 50 or 60 seconds' expo- 

 sure, if the room is darkened, the ex- 

 posed surface will be seen to emit 

 light. 



A set of tubes containing powders 

 which phosphoresce of different colours 

 can be obtained for about ten shillings ; 

 and if exposed to the same kind of 

 light (the lime-light will practically 

 answer if about 3 minutes' exposure 

 be given, or the tubes may be previ- 

 ously excited by sunlight) will glow 

 with their proper colours when the room is darkened. 



The connection between phosphorescence and fluorescence 



e E 



FIG. 174. Tyndall's Phos- 

 phoroscope 



