TOO E. M, NELSON ON EVOLUTION OF THE MICROSCOPE. 



C is to collect the light into a narrower compass where it falls 

 upon the object, after it has passed through a moderate hole in 

 the leather F." 



It should be noted that the leather F was only a friction washer, 

 it was not used for the purpose of a diaphragm at all, neither was 

 there any arrangement provided so that the size of the aperture 

 could be altered, or the leather changed. Consequently we must 

 come to the conclusion that a changing diaphragm was unknown 

 to Dr. Smith. The passage therefore in Part 1, p. 354, which 

 calls attention to Henry Baker's account in 1742 of the two 

 diaphragms in Wilson's screw-barrel microscope as being the first 

 notice of the use of diaphragms to regulate the illumination, 

 requires alteration. 



In 1710 we meet with a crude estimate of aperture, for Conradi 

 says that the aperture of his object glass was equal to a mustard 

 seed. This observer also used a negative amplifier between the 

 objective and eye-glass ; * this is the first notice of a Barlow lens. 



In the previous article we left off at J. Marshall's microscope 

 (1704), in which we found the important change from the 

 "telescope stand" to that of an inclinable limb holding both the 

 body and the stage. This is as decided an advance in the evolution 

 of the microscope as the advent of a vertebra is in zoological 

 evolution. In our next example, however, in so far as the mount 

 is concerned, we come to a throw-back to the older type, but in 

 other respects we find several important originalities. If we turn 

 to Fig. 12 of Hertel's microscope (1715) we shall see that the 

 mount is of the " telescope stand " type, the inclination in arc 

 being regulated by screw and nut. This mechanical contrivance 

 has for long left microscope workshops, but it is to-day very 

 largely employed in regulating the swinging windows in the 

 saloons of passenger steamships. 



The stage is like a circular table mounted on a central pillar. 

 Round the centre of the table are three circular openings, one of 

 which holds a black disc, one a white disc, the third being left 

 clear for the examination of transparent objects. Below this 

 clear opening is a plane mirror, an adaptation which survives to 

 the present day. The box foot does not contain the usual drawer 

 for apparatus, but holds machinery for the movement of the stage. 

 The butterfly nvits, which, strange to say, are placed in front of 

 * Bees' Cyclopa'dia : Art. " Microscope." 



