viii 



SCIENCE.- ADVERTISEMENTS. 



Ives^ Replicas of RowIand^s Gratings 



These replicas are made by a new process which gives gratings showing remark- 

 able definition in both the first and second order spectra. Even the smallest size shows in 

 direct sunlight all of the lines in Angstrom's map. The gratings are permanently 

 mounted between tested glass plates and are no more subject to injury than glass prisms. 

 They all have approximately 15050 lines to the inch. 



S. 111. IVKS GRATINGS with ruled surface about 3-4x1 iuch, fully utilizing the 



defiiiiiig power of spectroscoi^es of 1 inch aperture. Each . . . $8. GO 



S. 113. IVES GKATINGS with ruled surface 1 3-8x1 7-8 inches, fully utilizing the 



defliiing jiower of sijectroscopes of 1 1-4 inch aperture. Each . . $12.00 



S. 113. IVES GRATINGS with ruled surface 1 3-8x1 7-8 inches, specially selected. 



These specially selected gratings are almost absolutely equal in resolv- 

 ing power to an original grating of the same size, and will bear high 

 eyepiecing at full aperture in the second order spectrum. Each . $15 OO 



THE SCIENTIFIC SHOP 



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