36 Focal Length of Compound Systems of Lenses. [Sess. 



after being carefully washed in boiled water, were placed 

 in the sterilised test-tubes. The corks were reinserted, and 

 the tubes were suspended on the back wall of a very sunny 

 greenhouse, where they remained from April 1890 till now 

 (February 1892). The water remained perfectly fresh, the 

 fungal part of the lichen became colourless and died, while 

 the algal part got greener and grew considerably. The steril- 

 isation was, I believe, complete : all larger spores were re- 

 moved by the washing in the boiled water, while any bacteria 

 which might be carried into the fluid on the lichen were 

 probably killed by the constant exposure to the bright direct 

 sunlight. 



One of the most striking things to be noticed is the way 

 the lichen-alga withstands the direct rays of the sun. When 

 algas are cultivated artificially we know how essential it is to 

 shade them from the direct sunlight, or else they die in a few 

 days : the sides of the beakers in which they are grown are 

 usually covered half-way up with brown paper to make the 

 culture successful. A most distinguished chemist, who has 

 analysed different kinds of chlorophyll, says he has never 

 been able to analyse correctly the chlorophyll of cryptogams, 

 on account of its becoming so readily decomposed by light. 

 Here these little algal cells grew and prospered for a couple 

 of years, exposed to as intense a light as this climate affords. 



I have set up another dozen or more tubes, and I hope at 

 some future time to report to the Society the results. 



VIII.— ON SOME SIMPLE MEANS OF ASCERTAINING 

 TEE FOCAL LENGTH OF COMPOUND SYSTEMS 

 OF LENSES. 



By Mr WM. FORGAN, F.R.M.S. 

 {Read Feb. 24, 1892.) 



Mr Forgan first referred to the simple method of finding the 

 focus of a single lens, as described by the late Eev. T. W. 

 Webb in the first edition of his ' Celestial Objects for Common 



