E. M. NELSON ON HISTORIC MICROSCOPY. 225 



the objectives. After stating this fundamental principle he gets 

 a little hazy, for he goes on to say, " that the annular frame of the 

 lens must be very narrow, and connected to the microscope by two 

 or three slender wires or blades, so as to intercept as little sky- 

 light from the object as possible/' Of course, any light passing 

 through the object at a greater obliquity than the aperture of the 

 concave metal would have no effect at all. 



Although Sir D. Brewster passes such encomiums on this 

 microscope, and enters minutely into its construction, he leaves 

 these important principles untouched, not mentioning the illumi- 

 nating part of the instrument at all. 



As far as I am able to make out, Dr. Smith's microscope had a 

 power of 300 diam., and an aperture of about 60°. 



Lieberkiihn used single lenses, and illuminated opaque objects by 

 concave mirrors, 1738. 



The concave mirror, which was still used, was an advance, but 

 the single lens was retrograde ; the sliding focussing arrangement 

 was very crude. If these instruments were intended for use with 

 diffused daylight, the bi-convex condenser was a mistake, but it would 

 have been of advantage if a lamp were held in its principal focus. 

 About this time Culpeper made a compound microscope, vertically 

 placed on a box stand with a sliding-tube coarse adjustment, the 

 position of the body being indicated for the various powers after 

 J. Marshall's plan. Culpeper had no fine adjustment, but he 

 was the first to put a concave mirror for the illumination of 

 transparent objects. During the next 40 years the instrument goes 

 through great changes in the hands of Cuff, Adams (father and 

 son), Benjamin Martin, and Jones. 



In 1743 Cuff's microscope had a box-stand like Marshall's and 

 Culpeper's, a sliding coarse adjustment, with focus of objectives 

 marked as before, and a direct acting screw fine adjustment. A 

 diaphragm was here mentioned for the first time. The microscope, 

 however, did not incline. 



In 1746, Martin- Adams made a non-inclining microscope with 

 a rack and pinion focussing adjustment to the stage. The 

 various powers were fitted to a strip of brass sliding in the 

 nose-piece, with a notch and spring to centre them. It also 

 had a sliding tube focussing condenser fitted to the underneath 

 part of the stage, box-stand, plane and concave mirrors. 



In 1747 Cuff added a micrometer to his instrument, also im- 



