426 



SUMMARY OF CURRENT RESEARCHES RELATING TO 



sections of a flat spiral are made to act upon three sprung steel teeth, 

 causing them to project from slots within the cylinder, or to return to 

 their normal positions at will, B is a small ring with which each ob- 

 jective must be provided ; it screws on the objective, where it may 

 remain, and on its outer edge is a flanged groove. The objective, 

 having the ring B attached, can then be slid into the " Facility " nose- 

 piece, when about one-tenth of a turn of the milled collar on the 

 latter causes the teeth to grip in the flanged groove of B, thus securing 

 the objective in place ; the reverse movement releases the teeth 

 from the flanged groove, when the objective will drop into the 

 hand. 



As a piece of mechanism this device is ingenious. It appears to 

 us, however, that with high powers unsteadiness and defects of 

 centering will prevail to such an extent as to be fatal to its general 

 adoption. 



German "Cylinder-Diaphragms" and Condensers. — It is very 

 much to be desired that the old wheel of diaphragms so distinctive of 

 English instruments should be done away with, even with the smaller 

 stands. To replace it no expensive apparatus is necessary, the 

 " cylinder-diaphragm " of the German opticians serving all the 

 purposes for which the " wheel " is used, and having the advantage of 

 lying quite flush with the stage and exactly central, neither of which 

 points can be secured with the old form. The contrivance consists 

 simply of a tube a, a, fig. 74, having a narrow neck over which fit 

 small caps b, b (the exact size of the opening in the stage) pierced 

 with larger or smaller holes, c, as desired. 



Fig. 74. 



The usual condenser of the German instruments is also a very 

 simple and convenient apparatus for the smaller stands. It consists 

 only of a plano-convex lens I (fig. 75) in a fitting (a, a) nearly 

 identical with that of the cylinder-diaphragm holder. 



W. Behrens * considers the contrivance shown in fig. 76 a, a, with 

 three small movable disks, as convenient for use in combination with 

 these condensers for stopping off the central rays. 



For attaching the fitting of the cylinder-diaphragms to the stage 

 (as also the condenser) the slide ee, fig. 77, is useful and renders 

 unnecessary any alteration of the mirror, as is the case when the dia- 



' Hilfsbuch z. Ausfiihrung mikr. Untereuch. im, bot. Laborat.,' 1883, p. 69. 



