lloyal Microscopical Society. 215 



(Ic). Half-round hollows, with rod-Hke ridges in the middle; 



rounded interspace elevations somewhat lower than 



ridges and between them. 

 (2c). Narrower ridges ; nearly flat spaces. 

 (3c). Appearance of additional ridges, strongly shaded on left. 

 (4c). Narrow ridges, shaded on right ; flattish spaces, and low 



ridges, with narrower shelving shade-spaces down to 



ridges, &c., &c. 



Same condenser 109°; one radial slot, which was rotated to 

 various angles. Angle of mirror varied. 



(Id). Each cut made into a flattish space, with two narrow, 



raised edges, shaded on left. 

 (2cZ). Cuts made into flattish, ribbon-like elevations, with raised 



edges. 

 (3(?). Interspaces raised, with rounded edges ; cuts made to 



look flattish, and at lower level. 

 (4^). Appearance of additional and imperfect ridges. 

 {bd). Series of imbricated and shaded bands. 



In the lines cut by Mr. Ackland no attempt was made to pro- 

 duce the narrowest possible furrows. The width of furrows found 

 practically convenient for micrometers was only slightly deviated 

 from, as some cuts were a little deeper than others, and thus caused 

 the wedge-shaped diamond-point to open the top of the furrows a 

 little wider. The interspaces of the narrowest were much wider 

 than the cuts. It is obvious that a cut wide enough to be distinctly 

 seen, under given magnification, will present to view hvo linear 

 edges, and thus be reckoned as hvo lines if its true character is not 

 considered. Cuts very close together may, if the cohesion of the 

 glass and the perfection of the cutting tool permit, be wider than 

 their interspaces. 



It will be seen that in the preceding statements only one 

 instance is mentioned of appearances agreeing tolerably well with 

 the real facts. It must not be inferred from this that it is not easy 

 to exhibit moderately-fine cuts correctly, or very nearly so. The 

 object of this paper was to select a number of appearances all look- 

 ing as if they might correspond with the facts, and all differing 

 more or less from them. 



Those who study the most vexatious diatoms or Nobert's test- 

 lines must, it appears to the writer, not only take into account 

 what they do see, but what they ought to see, provided the object 

 has a certain definite structure, and certain powers of producing 

 optical images under given conditions. 



VOL. V. 



