322 



ACCESSOKY APPARATUS 



objectives equal to real apochromatics, and it sharpens the images 

 yielded even by the latter, whilst it increases resolving power in all 



lenses, arid amelior- 

 ates the strain often 

 felt by workers who 

 have not before used 

 it. 



The cell contain- 

 ing the solution and 

 worked glass may 

 either have its upper 

 end sealed hermeti- 

 cally with paraffin, or 

 be simply carefully 

 corked ; the latter 

 plan, if the cork is 

 carefully made, ad- 

 mits of the easy 

 opening of the cell 

 and renewal of the 

 fluid. A diagram- 

 matic illustration of 

 the effect of the use 

 of the screen is given 

 in fig. 267, which 

 represents the band 

 of colour passed 

 through the F-line 

 screen. The green 

 is represented by the 

 horizontal lines, and 

 the blue, in which 

 the F line is situated, 

 by the diagonal lines. 

 The cell itself is 

 prepared by the Ley- 

 bolds process, and is 

 fused at the joints 

 and never leaks ; a 

 still simpler and less 

 expensive means of 

 making such a filter 

 has been devised by 

 Dr. A. Meithe, pro- 

 fessor of spectral 

 analysis at Berlin. 

 The filter consists of 



_ a trough containing 



| of an inch in thickness of saturated solution of acetate of copper 

 filtered ; a variation in the thickness of the troughs or tanks 

 desirable, but the results are excellent. 



