320 



tre of spherical motion. I showed models of this at the time to va- 

 rious friends, but I never proceeded to the construction. Since that 

 time many exceedingly ingenious arrangements for so-called oblique 

 illlumination have been brought forward ; for instance, by Thomas 

 Ross, Wenham, Shadbolt, Amici, Nachet, Nobert, Topping, and 

 others, all of which are so fully described in Queckett's admirable 

 work that it is unnecessary to do more than refer to the volume. With 

 the exception, however, of Nachet's oblique prism, all these ar- 

 rangements are in fact for direct illumination, stopping off the cen- 

 tre of the cone of rays before alluded to, and thus leaving the field 

 of the instrument dark, little or no light reaching the eye but that 

 coming from the object under examination ; but as this light reaches 

 the object equally from all azimuths, there are not any shadows 

 the formation of which I conceive to be the distinguishing characte- 

 ristic of oblique illumination. 



"Among several of these illuminators, I procured during the last 

 summer that which is known as the Paraboloid. This produces an 

 annulus of light with a dark centre, but as it (like most of the 

 others) throws the light obliquely but uniformly on every side of 

 the object, there are, of course, no shadows. It occurred to me 

 that by a slight addition this instrument offered the means of test- 

 ing my old theory of oblique illumination with variable azimuth. 

 The instrument consists of a solid paraboloid of glass, with a plane 

 base, the focus near to the summit, which summit is ground away 

 to form a spherical cavity, the centre of which coincides with the 

 focus of the parabola ; the middle of this spherical cavity is fur- 

 nished with a dark stop ; the action is, that parallel light falling per- 

 pendicularly on the base passes into the glass without refraction, 

 and from the inner surface is reflected to the focus, which it reaches 

 through the sides of the spherical cavity, also without refraction. 

 The effect is, as I have before said, a speck of light at the focus, 

 which is unfolded above or below into an annulus of light, with a 

 dark centre ; the object being adjusted perfectly coincident with 

 this focal speck, is seen by the light radiating from its surface as 

 before described, but the field is devoid of light. 



" My addition to this is a glass prism producing two internal 

 total reflections at right angles to each other ; this prism is fixed to 

 a disc of brass below the base of the paraboloid, and prevents any 



