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HI. — On a Method of obtaining Oblique Vision of Surface 
Structure, under the Highest Powers of the Microscope. 
By F. H. Wenham. 
{Described before the Royal Microscopical Society, March 3, 1875.) 
If we take a thin semi-transparent object, such as a slice of melon, 
for the purpose of examining its structure, instinct directs us to 
hold it aslant to the light, and also to view it in an oblique direc- 
tion. Experience has taught us that if we oppose the object to 
very strong rays, and look straight through it, the minutiae of con- 
figuration will be obliterated by a flood of light. It is in this way 
that we see objects under the microscope, as their mounting lies 
square with the axis of the object-glass. 
For resolving the striated structure of the most difficult tests, 
extreme obliquity of the illuminating pencil is requisite, and 
numerous adaptations to the microscope have been invented for this 
special purpose. Let angle a, Fig. 1 , represent the aperture of the 
object-glass, b the angle of the illuminating pencil for an object in 
the focus. It is evident that the portion of the object-glass brought 
most strongly into action under these conditions, is that opposite to 
the direction of illumination ; the near extreme will he in compara- 
tive darkness, and it is questionable whether this gives any material 
aid, by collecting mere radiations necessarily feeble, from surface con- 
figurations. This favours the hypothesis that, besides oblique light, 
oblique vision is an important condition for discovering the closest 
striations on microscope tests, and that their development does not 
probably depend so much upon a large aperture as the degree of 
obliquity at which they are viewed. Oblique vision is thus obtained 
by the extreme marginal and most imperfect image-forming rays of 
the object-glass. The best or central ones come so little into action, 
that they even mar the result ; for, if obscured by a central stop, 
the exterior light is not overpowered, and greater distinctness in 
