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XIII. — A short Description of M. Nachefs Prisms, for the Oblique 

 Illumination of Transparent Objects on the Stage of the Micro- 

 scope, with Formula for their Construction and Observations on 

 the their Use. By George Shadbolt, Esq. 



(Read March 20, 1850). 



The use of some contrivance for causing the light to fall obliquely 

 upon an object under examination, must be familiar to all micro- 

 scopists who have been engaged in investigating the structure of 

 such as have very minute and delicate markings, that are difficult to 

 be defined ; very slight eminences and depressions, as well as some 

 other peculiarities of structure, being thus rendered more apparent 

 from the increased contrast of light and shadow. 



In order to render such a mode of illumination effective, it is, 

 however, necessary that the rays of light should fall in some parti- 

 cular direction with respect to the object ; and to accomplish this 

 requisite with facility, a modification of the stage generally attached 

 to our modern microscopes was arranged and carried out by Mr. 

 Legg. 



In the address delivered by our late respected President, Mr. 

 Busk, at the anniversary meeting of this Society, in February last, 

 he mentioned, when enumerating the additions to our means of 

 investigation, the production, by M. Nachet, of Paris, of a prism for 

 the purpose of effecting oblique illumination, which little instrument 

 has also some advantages over the revolving stage. As I have found 

 from practical experience that it is of very material assistance in 

 investigating the structure of the class of objects above indicated, I 

 have presumed to think that a short description of it, together with 

 data for its construction, may not be without interest to some of our 

 members. 



The first instrument of this kind that I met with was shown to 

 me by one of the members of this Society, Mr. Hodgson, who 

 brought it over from Paris, and I afterwards saw some that Mr. De 

 la Rue obtained from the same quarter. 



The prism consists of a piece of glass with six faces, two only 

 being parallel to one another, and these two sides are not polished, 

 in order to prevent any reflection. The other four sides are inclined 



