326 
SUMMARY OF CURRENT RESEARCHES RELATING TO 
pulled out for a distance of two or three inches ; there is a line drawn 
vertically down this third tube, and a mark engraved on the second 
tube ; when this line is opposite the mark, it indicates that the body is 
in a line with the centre of the stage. Here the pulling or pushing tube 
acts as the coarse, and the rackwork as the fine adjustment. The stage 
is fixed, but both “forward” and “motion in arc” over the stage are 
given to the body. These motions of the body over the stage were 
derived from Ellis’s Aquatic Microscope, made by Cuff in 1755, and were 
highly thought of at that time. The last Microscope so fitted was one 
designed by Mr. W. Valentine, of Nottingham, and made by Andrew Ross 
Fig. 73. 
in 1831. The motion in arc was however not given up until later, and 
to a partial extent still survives in Powell’s No. 1. 
With regard to the optical portion of this instrument, we find an 
elaborate eye-piece. It will be remembered that it was pointed out in 
the description of a Benjamin Martin Microscope last year,* that the 
eye-lens was broken up into two lenses of equal foci, the first being a 
crossed, and the other a plano-convex lens. In this one we have a 
further development ; for the plano-convex lens is again broken up into 
two plano-convex lenses of 3 in. foci. The equivalent focus of 
these three eye-lenses is 1 in., that of the field-lens 3 in., and the 
* Cf. Journ. R.M.S., 1898, p. 474, fig. 81. 
