Transactions of the Royal Mieroscojaical Society. 241 



takes place, opening out as it were into detail the form of bosses or 

 ribbings. On that superb test, the Podura, for example, when the 

 light is thrown from the apex to the quill, the whole scale is dotted 

 over with bright blue spots laying in a zigzag direction ; these are 

 the most jDrominent parts or the club-end of the markings, which 

 are nearest in contact with the glass. As the light continues to 

 revolve, these gradually arrange themselves in the usual note of 

 exclamation figures. In the reverse direction, or from quill to 

 point, the blue spots are rather less distinct, and the object is most 

 brilliant when the light is transverse to the scale. By making the 

 light as oblique as possible in a direction from the point to the 

 quill, I have for the first time been able to get a satisfactory view 

 of the surface tissue of the Fochira ; it appears of a pale brown 

 colour in waves or transverse undulations which reflect light visibly, 

 and follow the zigzag of the markings. I need not, however, at 

 present describe effects that will soon be known, further than to 

 remark that Amjohij)leura iieUucida assumed a substantial appear- 

 ance, not seen in any other way, and at once displayed its striae 

 with an ^th that had never resolved them before. As partly 

 accounting for these efiects, the black field has a material influence, 

 and it has been long known that the resolution of very difficult 

 striae depends upon excessive obliquity of fight. Now the most 

 oblique that we can obtain by ordinary means, cannot strike the 

 object at an angle greater than 41°, if it is either in balsam or on 

 the slide, but on this principle we are dealing with rays entirely 

 beyond this angle. 



I will venture to anticipate a few objections that may be made 

 to the arrangement. One might be that it is not achromatic. This 

 would increase the expense, and I do not think will have the 

 smallest advantage. The light spot is now so definite that its 

 margin can be brought half across an Angulatum scale, at which 

 there is a chromatic efiect produced. This is by no means detri- 

 mental — quite the reverse, as some fine appearances are produced 

 by it. 



It may be said that the rays reflected from the lower end of the 

 facet are just without the angle of total reflexion, and might enter 

 true, and I had intended to stop ofi" a small segment of the lens at 

 this place, but found it so desirable, in man}' objects, to admit a little 

 light, that I preferred it without alteration. It is easy to get a 

 black field in all cases by mere mirror adjustment. 



Also it might be objected that the light is always one-sided, 

 which in many objects is well known to produce a host of false 

 appearances ; but this does not apply in this principle of illumination. 

 It seems sufficient to get light into the object alone, whether side- 

 ways or not, so as to make it appear as a self-luminous body ; and 

 there is none of that coma thrown off like an overhanging veil, or 



