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equilateral sides rendered concave for the reception of a 

 double convex lens of crown glass, and mounted in such a 

 manner that to it every possible position can be given. In 

 the prism, as constructed by Abrahams, of Liverpool, the 

 focus of the lens is 4 inches. In that as constructed by 

 Powell and Lealand, 2^ inches. I do not know that there is 

 any choice between them ; the selection should dejjend more 

 on the convenience with Avhicli the one or the other can be 

 used, in connection with the stage of the instrument. 

 Although admitting, if desired, of being mounted upon the 

 mirror arm of the microscope in lieu of the mirror, still you 

 will find it preferable, and affording greater facility of 

 manipulation when mounted upon a separate and indepen- 

 dent stand, consisting simply of a perpendicular rod with 

 foot, as in the case of the ordinary bulFs-eye condenser. The 

 English mode of mounting, however, upon such stand is not 

 as perfect as it might be, and is far excelled by that of Mr. 

 George Wales, a member of our society. In use parallel 

 rays, and if by artificial light, rays rendered sufficiently 

 parallel for the purpose by removal of tne light some twenty 

 or more inches from the prism, in preference to the inter- 

 position of a lens or lenses, v^ith a view to the same effect, 

 are caused to fall upon the surface of the lens, and being 

 converged then upon the diagonal surface of the prism, total 

 reflection takes place, and emerging at right angles from the 

 remaining side of the prism, the rays are brought to a focus 

 at a point beyond or distant from the object, dependent upon 

 the power of the objective. By the combination of flint and 

 crown glass, achromatism is secured, and the total reflection 

 which takes place from the diagonal side of the prism not only 

 enables us to avoid the secondary reflection present upon the 

 use of the mirror, but, as is well known, gives a purity and 

 intensity of illumination unexcelled. 



By such mode of illumination one is enabled, e. g. with a 

 l-5tli in. objective, to bring out the markings upon the 

 Podura scale, with their central light streak or interspace 

 Avith a sharpness, clearness, and depth of definition unex- 

 ampled, and leaving nothing more to be desired. 



A word in reference to a want of care in central illumina- 

 tion before leaving this part of our subject will probably be 

 excused, and may not be out of place. I refer to the too 

 frequent use of a beam of light, not exactly centric, or, in 

 other words, not truly coinciding with the optical axis of the 

 instnnnent. This is easily determined by the equality in 

 width of the " diffraction band" encircling the periphery of 



