Notes. 



567 



front of the eyepiece. One is fixed and the other can be tilted about an axis 

 perpendicular to the line of division of the plates. 



I have used both forms, and I find both of them exceedingly convenient 



/<f 



Fig. 132. 



rapid, and accurate. The parallel plate micrometer is easily constructed, 

 and is inexpensive. It merits more notice and much more use than it lias 

 yet received (Fig. 132). 



The Polishing of Glass Surfaces. 

 By Lord Rayhigh, OM. F.B.S. 



Theoretical treatment of the question consisted in the inquiry into the 

 difference between reflection from a perfectly plane surface, and from one 

 which had corrugations. This difference depended entirely on the relation- 

 ship between the period of the corrugation — i.e. the distance from ridge to 

 ridge along the surface, and the wave-length of the vibration that was being 

 reflected. The question in connection with gratings had been treated long 

 ago by Fraunhofer, who had drawn conclusions as to the limits of the power 

 of the Microscope, from the fact that when the lines of a grating an' closer 

 together than a wave-length, the spectra which would be formed if the 

 grating had been less closely ruled were, so to say, pushed out of the field. 

 With the slight correction that Fraunhofer had not treated the case of 



