214 Transactions of the 



usually filled up with finely-divided black-lead, and the same mate- 

 rial has been employed in the writings and patterns made with the 

 Peter's machine. This substance of course modifies the appear- 

 ances. To see them in a simpler form recourse was had to Mr. 

 Ackland (Home and Thornthwaite), who ruled several sets of fine 

 lines, each on glass slides, at varying distances, 1 — 2000", 1 — 3000", 

 and 1 — 4000", and mounted them with Canada lalsam, so that 

 they could be safely used with immersion lenses. One set was not 

 covered or mounted in any way. 



Those who have examined very minute writing done by the 

 late Mr. Farrants with the Peter's machine will be aware that even 

 when a very fine diamond-point is used, the incision partakes more 

 of the character of a scratch than a clean cut. It seems impossible 

 to cut glass with a smooth, clear edge, such as certain metals readily 

 give with a sharp tool. A hne cut in glass is thus a furrow, more 

 or less rough at the bottom and sides, and when viewed correctly 

 under the microscope, has the appearance of a narrow depression 

 less transparent than the ad.jacent spaces. It is difficult to get a 

 really correct view. Even under favourable circumstances of illu- 

 mination and correction, the edges of a cut are apt to appear as two 

 raised hues. 



Many instructive optical appearances, which might bewilder the 

 observer if the true character of the object were not known, may be 

 easily produced, as the following notes will show. The observations 

 were made with Powell and Lealand's immersion ^th and Boss's 

 xVths, condenser aperture 109°. Using central stop A, and vary- 

 ing inclinations of mirror. Paraffine lamp. 



(la). Cuts as rounded bands ; interspaces flattish furrows. The 

 bands illuminated on right side, shaded on left. Tint 

 of lightest part of furrows bluish. 



(2a). Flattish bands and rounded furrows, the former slightly 

 shaded on left ; tint of shading bluish. 



(3a). Oblique rounded furrows with narrow blue ridges ; 

 broadish bands with narrower elevated bands up their 

 centres, light on right side, shaded deeply down the 

 furrowed side on left. 



Same condenser 109°, two radial slots forming obtuse angle. 

 Angle of mirror varying. 



(lb). Broad, flat spaces, narrow, shaded, and elevated ridges. 



(2i). Ptidges four times as wide as No. 1, with rounded tops. 



(3&). Narrowish grooves, something hke actual object. 



(4&). False ridges, puzzling to count, and hollow. 



Same condenser 109^, two rectangular radial slots. Angle of 

 mirror varied. 



