The Microscope. 209 



The application of the prism will not, of course, make an objective 

 resolve a test beyond the reach of its aperture; but it often happens 

 that in the case of close-lined objects we can see the spectrum at the 

 back of the objective when the lines cannot be seen in the object 

 itself. It is then that the prism shows its power, as its use will at 

 once bring out the lines with the greatest ease and sharpness. 



Mr. E. M. Nelson found, while investigating the matter, that the 

 diffraction spectrum of A. pellucida (illuminated by oblique beam 

 from oil-imm. achromatic condenser, and with a water-imm. yV), 

 showed all the green, but no red. On examining the spectrum 

 through the analysing prism without an eye-piece, he found that 

 when the prism was in a line with the dioptric beam and the difPrac- 

 tion spectrum, the brightness of the green was intensified. On 

 replacing the eye-piece, and viewing the image through the prism 

 used above the eye-piece, as directed by Mr. Hirst, there could be no 

 doubt that the transverse striae were much sharper and blacker than 

 when viewed without the prism. The prism must, of course, be kept 

 in a line with the dioptric beam and the diffraction spectra. Should 

 the prism be turned across, even if it does not cut off the aperture, 

 the definition will be impaired. 



He next changed the water-imm. -^^ for a water-imm. -^^ of less 

 angle, which would barely resolve the A. pellucida — that is to say, 

 would only resolve it in patches, and not from end to end. On 

 examining this with the prism, he found that the parts which were 

 unresolved were still unresolved; but those parts which were resolved 

 were intensified. 



The image of A. pellucida, with an apochromatic ^ (4 N. A.), 

 my new eye-piece, and the prism is something very fine, such as I 

 have never seen before. 



He also tried the prism with several very subtle direct light 

 tests, but cannot say that he found any improvement in the image. 

 On the whole, he should think this class of objects would be seen 

 better without the prism. Probably the e£Eicacy of the prism, when 

 used with a lined test, lies in the fact that it intensifies the diffraction 

 spectra when it is placed in a certain direction to it. 



— Koy. Mic. Journal. 



Pond Dredging and Collecting. — In reply to a correspondent's 

 inquiry in Hardivick's Science Gossip, as to the use of nets in col- 

 lecting rotifera, etc., Mr. Jno. Eyre writes: I should advise him not 

 to use a net at all. The best appliance with which I am acquainted 



