Method of Reducing Prismatic Spectra. 211 



It may be useful here to describe in detail the exact method 

 of procedure finally adopted. It has not been found neces- 

 sary to use optically worked glass ; good ordinary plate-glass 

 gives perfect results. Sextant glasses have been recommended 

 to us for this purpose. It is well to select two plates having 

 the most suitable surfaces. This can be done by placing one 

 plate on another, the two adjacent surfaces having previously 

 been cleaned with cotton- wool, and viewing the air-film 

 between them by reflected light from a sodium flame. The 

 bands formed when the plates have been gently pressed 

 together should be nearly straight and each one at least 

 2 millim. or 3 millim. in breadth. 



The two selected surfaces should now be silvered somewhat 

 heavily. The milk-sugar process"*, in which the silver is 

 deposited very slowly, has been found to give good results. 



A simple mechanical arrangement for adjusting the two 

 silvered surfaces for parallelism, the distance between them 

 being also capable of adjustment, could easily be designed. 

 We have found, however, that if a little soft wax be placed 

 round the edges of the plates a perfect adjustment can be 

 obtained by simply pressing the plates together with the 

 fingers. The photograph accompanying this paper was ob- 

 tained using this arrangement. To adjust for parallelism, 

 view a spot of light, or the filament of an incandescent 

 electric lamp, through the silvered surfaces. A long train of 

 images, due to multiple reflexions, will generally be visible. 

 These images having been brought into coincidence, inter- 

 ference-bands will generally be seen on viewing a sodium 

 flame through the silvered surfaces. These are adjusted, by 

 pressure applied to the glass plates, to be as broad as possible. 

 When the adjustment is nearly completed there is often some 

 difficulty in seeing the bands, due to the fact that for a 

 parallel air-film viewed normally the interference-bands are 

 formed at an infinite distance in front of the filmf. At this 

 stage the bands should be viewed from as great a distance as 

 possible. The perfection of the results finally obtained will 

 depend greatly on the accuracy with which this adjustment 

 is performed. 



If the collimator slit of the spectrometer be now illumined 

 by a slightly convergent beam from an arc-lamp, and the 

 plates be placed in front of the slit, and as near to it as 



* For the exact process employed by us see { Nature,' Sept. 23, 1897, 

 "On the Phase-change of Light when Reflected from a Silver Surface," 

 by Edwin Edser and H. Stanstield. 



t A. A. Michelson on " Interference Phenomena in new form of 

 Refractometer," Phil. Mag., April 1882. 



