S8 



i-KOI'lMlTIKS Ol' ni-IICAI, (;UATIN<;S. 



slit. B is a short focus k-ns. It nnist be well correctL'd for 

 chi-omatic aberration, as it must work with a fairly large 

 diaphrasini. A synuiietrical camera lens by Beck of about 

 2S cms. focal length, working at FH, proved very satisfactory. 



n 



-— H- 



CD 



T^u 



re 



C is another vertical .slit. Its jaws must be good, and it 

 should be fitted with a micrometer adjustment enabling its 

 width to be measured. Otherwise, some such device as 



closing it on a piece of metal foil whose thickness is after- 

 wards measured by a micrometer must be employed to de- 

 termine its width. D is a wire grid, made by soldering 

 togethei' a metal rectangle, two sides of which are screws of 

 tine pitch. Twenty-two gauge copper wire is wound round 

 the rectangle over the screws, thus foiniing a double grid of 

 the same pitch as the screws. The wire is next fa.stened to 

 the rectangle with sealing wax, and the strands on one side 

 cut away. In this way a grating of 22 lines, of pitch .7(5 mm., 

 was nuifle. and ))r(>ved satisfactoiy. E is a telescope lens, 

 corrected foj- (hronjatic a'lerration. of about a nu'tre ftx-al 

 length. I' is a micrcmeter eyepiece in whose lield the 

 j)henoinena to be described are observed, and by means of 

 which distances on the (liffraction patterns are measured. 

 An ordinary eyej)icce and scale could probably be made to 

 serve. 



.^KCTION 2. AD.irSTMFONT. 



A is opened to about a third of a millimetie, and B ad- 

 justed to render light from A |)ar«'llel. (' and I) a<-c re- 

 moved, and F. and F so arranged that A is sharply focussed 

 along the vertical crosswire of F. I{, K. and F should be 

 sufficiently well corrected to give a brilliant image of A with- 

 out strikintc aberrations. 



