524 Dr. G. J. Stoney on Microscopic Vision. 



objective. This has been sufficiently illustrated in § 35. The 

 remedy is to insert a stop of the proper size. A similar fog 

 of light will occur if, by opening the iris diaphragm too far, 

 diffracted light has been rendered too copious in situations 

 where it is beyond the grasp of the objective. Each part of 

 it will then produce its own body of intercostal markings ; 

 these get massed together and produce a haze. The remedy 

 is of course to lessen the opening of the iris diaphragm*. 



A case which should be specially noted is that of a bar on 

 the object with sharp edges. These produce highly inclined 

 diffracted beams ; and if the objective cannot take these in, 

 the beams — B6 of p. 344 which must then be added to 

 standard image No. 1, round off the edges and add thin 

 appendage-lines which are often mistaken for diffraction- 

 fringes though they have a different appearance and a quite 

 different origin. 



41. " Optical Contact: 3 See § 19, p. 348.— An excellent 

 illustration of the effect produced by Stokes's layer — the 

 effect which microscopists call optical contact — may be 

 obtained as follows : — 



Focus a valve of Pleurosigma angulatum, mounted dry, 

 under an immersion-lens of, say, Gr (or NA)=1*3. Adjust 

 the draw-tube. Then set up the apparatus described in § 35, 

 p. 12, for furnishing monochromatic light, and select green 

 light of about wave-length A, = 0'55 fi. Now 0*55 ft is also 

 the value of a upon this diatom, i. e. it is the spacing asunder 



Image x, with X scale. 



of its rows of markings. Hence from the formula 

 <r=\/(g+g / ) we find that g-\-g' = l, where g' is the grasp 

 (or radius in image x out to the punctum) of a dioptric 



* A haze of light may have a different origin when the section is too 

 thick. In this case the parts that are out of focus produce it, and it 

 may often be cleared up without detriment to the observation we want 

 to make by partially closing the Davis's shutter. 



