1050 SUMMARY OF CURRENT RESEARCHES RELATING TO 



pin, shown in the fig. There is also a small wooden stage on 

 which materials, &c., can bo placed, or a bottle or test-tube can be 

 stood upon it and held in place by boing tied to a rod which slides 

 vertically in the brass socket supporting the stage. The carrier on 

 the left is provided with a long spring, under which rods of various 

 shapes for holding objects can be slid and rotated in notches and 

 holes made on either side of the fork-support. 



This is the first application known to us of ball-and-socket move- 

 ments to a simple Microscope. 



Beeldsnyder's Achromatic Objective.— Another object of interest 



brought by Prof, Iliibrecht, was the objective shown in fig. 218, which 



is of special interest in the history of the evolution of tho Microscope 



from the fact that the late Prof. P. Harting * assigned 



Fig. 218. ^s construction to about the year 1791, by Francois 



. jg r jfc Beeldsnyder, of Amsterdam. 



•^S»ri_ __. v The combination consists of two bi-convex (green) 



crown lenses of 22 mm. and 19 mm. focus respectively, 



with an interposed bi-concave flint lens, the combined 



focus being 21 mm., and the diameters 6*5 mm. The 



Si ~~m lenses fit somewhat loosely in a brass cell screwing 



JfisgJI' into the brass mount. The surfaces are somewhat 



imperfectly polished. The image obtained by the 



objective when used with an eye-piece, is but little better than that 



given by an ordinary non-achromatic simple object-lens diaphragmed 



as was usual before the application of achromatism. The increase of 



light due to the greater aperture hardly compensates for the loss due 



to the greyness of the polish. 



Queen's "Parfocal Eye-pieces." — Messrs. J. W. Queen and Co. 

 announce f that they are prepared to furnish eye-pieces (parfocal = 

 of equal focus) which can be " changed without altering focus," or, 

 in other words, eye-pieces with which the amplification of the Micro- 

 scope is in exact inverse proportion to their focal length. This is 

 accomplished by so adjusting the mounting of the eye-pieces that 

 their anterior principal focus always lies at the same place in the 

 body-tube. 



The position of the anterior principal focus is readily calculated 

 for every eye-piece. If a is the distance of the diaphragm from 

 the field lens, and x the focal length of the latter, the distance of the 

 anterior focus above the diaphragm will be 



p = _«L = »Vl + «). 



x — a x\ xJ 

 strictly approximately 



* l Das Mikroskop' (German trans.), 2nd ed., 1866, iii. pp. 132-3. 

 f Micr. Bulletin (Queen's), iii. (1886) p. 31. 



