ZOOLOGY AND BOTANY, MICROSCOPY, ETC. 
205 
like a Cassegrainian telescope, with this difference, that both mirrors are 
pierced with a hole through their centres. Dr. Smith calculated four of 
these Microscopes, the main difference between them being in the amount 
of spherical aberration they possessed. He also describes a substage 
condenser. This Microscope was not surpassed either theoretically or 
practically for about eighty or ninety years. 
1738. Culpeper and Scarlet’s “Double Reflecting Microscope” is 
the first instance of an English Microscope with an illuminating mirror 
(fig. 36). Baker, in his description of this instrument in 1743, adds a 
Fig. 36. 
oonical diaphragm of black ivory (fig. 37). A brass diaphragm of this 
form was subsequently made by Cuff, Marston, Adams, and in 1798 by 
Jones. Chevalier placed a graduated wheel of diaphragms at the lower 
end of the cone (1823), so that this peculiar form of diaphragm had an 
innings of about eighty years. 
The Culpeper and Scarlet model remained the popular form for 
seventy years. 
1738. In this year Dr. Lieberkuhn introduced his well-known form 
of reflector for the illumination of opaque objects ; fig. 38 is a copy of 
a figure in a book by P. van Musschenbroek (1739). 
