64 J. EHEINBEHG ON A DIFFRACTION SPECTRA OCULAR. 



at the upper end, and then focussing down on to the upper focal 

 plane of the objective, but this is a cumbersome business. 

 Latterly I have employed a method which is exceedingly conveni- 

 ent and efficient. The diffraction spectra, as is known, are not 

 only formed in the focal j)lane of the objective, but are reformed 

 just above the eyepiece, and may there be viewed by means of a 

 lens. So I have mounted in a short tube the objective of one of 

 the 7s. 6d. toy microscopes, Avhich is in effect a lens of about a 

 I inch focus stopped down to an actual aperture of about 1 mm. 

 The accompanying diagram will explain itself. The diffraction 

 spectra ocular, as we may call it, when placed on the ordinary 

 eyepiece of the microscope, on the cap of which it fits, shows the 

 spectra sjDlendidly, magnifying them at the same time. It gives 

 plenty of light, and the spectra cannot shift. I can strongly 

 recommend the arran element to those who cultivate the useful 

 habit of studying the spectra, i.e., the optical effect produced by 

 an object, as well as the object itself. 



Journ. Quekett Microscopical Club, i'er. 2, Vol. VIII., No. 48, A2'>ril 1901, 



