394 PHOTOGRAPHING MINUTE OBJECTS. 



est the light, should be as large as possible, so as to pass as much 

 light as you can through the object. The analyser follows on at 

 the back of the objective, and should be mounted as close to the 

 back lens of the objective as possible, otherwise it will cut off some 

 of the field, and, to get the best result, both the polariser and 

 analyser should be made to rotate. A secondary condenser may 

 be placed in front of the Nicol with advantage. 



Work done by dark ground illumination has a great charm, 

 more particularly to the student who has a love of pond life, and 

 to see the minute organisms scudding about lit up like small par- 

 ticles of silver on a black ground. The common hydra, vorticella, 

 and many others may be photographed by one who gives it his 

 careful study. Diatoms, sponge spicules, polyzoa, and many other 

 objects look grand done in this way. The few slides that I shall 

 show I hope will bear me out. I fear that I have taken up a lot 

 of your time, and you are getting tired of this dry stuff ; there are 

 several other matters that I might have dealt with, but it would 

 take up too much time in the one evening. 



The Rays of the Solar Spectrum. — The fact is well known 

 that if we examine the spectra furnished by the light emitted by 

 the various points of the sun, the rays that appear are very variable 

 in number. There exist but eleven that are constant — that is to 

 say, that we find in the light derived from all the regions. Among 

 these, five belong to hydrogen, two to calcium, and four to 

 unknown elements. Mr. Ramsay, however, has identified one of 

 these rays — that of helium, with the ray of a terrestrial element. 

 There remained then but three, corresponding to extra terrestrial 

 substances. Mr. Deslandres has decomposed clevite by sulphuric 

 acid, and then, on studying the spectrum of the gas disengaged, 

 has ascertained the existence or a ray 447 "i 8, identifiable with 

 one of the three remaining rays. In consequence of this dis- 

 covery, there exist but two unknown rays among the permanent 

 ones of the spectrum. — Set. American. 



