Francis Watkins' Microscope. By E. M. Nelson. 141 



has now a much stouter form of joint,* and the point of its attach- 

 ment is in the best position for stability. The plate by which the 

 body is attached to the limb has a strengthening bracket below it. 

 One cannot help thinking that the noble designer of the " Variable " 

 Microscope must have been acquainted not only with this design of 

 Watkins', but also with its faults, which he specially corrects while 

 following the Watkins' design in the main. 



Eeturning again to Watkins' Microscope, we find the wheel of 

 powers much improved. The seven | powers are mounted between 

 two disks of silver 1 ■ 15 in. in diameter. This form of the wheel 

 of powers lasted until the early part of the nineteenth century, for 

 it was afterwards adopted by Adams, Benjamin Martin,! and still 

 retained in the " Most Improved Compound " Microscope of Jones 

 in 1798. 



If a digression is allowed, it may be explained that the 

 nobleman's " Variable " was optically of a very advanced type. 

 The Huyghenian eye-piece had, in addition to the field-lens, a 

 double eye-lens ; there was, besides, another lens lower down the 

 tube, to act as a back lens for the various powers — this was probably 

 copied from Benjamin Martin. § The " Variable " had a very im- 

 portant novelty, for the powers were not placed in a wheel, but 

 were mounted in separate " buttons," so that they could be com- 

 bined, which was of course a great advance, for by this means the 

 spherical aberration was reduced, and so a larger aperture could be 

 used. The nobleman's " Variable " was therefore the first Micro- 

 scope to possess an objective which was a " combination." If any 

 one takes the trouble to examine a good specimen of an old non- 

 achromatic Microscope, they will find that the image, field, etc., are 

 not at all bad, so far as they go : the one drawback is lack of aperture. 

 The spherical and chromatic aberrations were so great that the 

 apertures of the object-glasses had to be reduced to a pin's point. 

 The fault, therefore, with all of them is too much empty magnifica- 

 tion. 



The best form ever attained in pre-achromatic days was either 

 Wollaston's doublets (1829) or Coddington's Microscope (1830). 

 These instruments will show the watered-silk appearance upon a 

 strongly marked Podura scale just breaking up into small exclama- 

 tion marks. 



* Joints of this form were in common use for Gregorian and other telescopes 

 at that time. 



t Lindsay's Microscope, patented 1743, had seven powers mounted in two 

 strips, four in one, and three in the other. 



X At the sign of Hadley's Quadrant and Visual Glasses, near Crown Court, 

 Fleet Street. 



§ I have made exhaustive experiments with Martin's back lens, and find that 

 it is an advantage because it increases the N.A., and still more the Optical Index, 

 as it lowers the power. The focal length of the lens is 5| in. See this Journal, 

 1898, p. 474, fig. 81. 



