38 PRACTICAL TREATISE ON 



milled head, g, this formed the coarse adjustment, the lower 

 end of this tube, e, was conical, and to it the object-glasses, f, 

 were screwed. The third or outer tube was firmly fixed to 

 the arm, a, by a curved plate of brass and by the screw, c. 

 When the compound body was placed in the inclined position, 

 as represented by the figure, the tubular rods, d d, were used 

 to steady it, the nuts, d d, serving to fix them when the proper 

 inclination had been obtained; these rods were attached to the 

 two hindmost feet. When the draw tube was in use, it could 

 be fixed by the moveable band surrounding the body, and 

 having a clamping screw, j. To the stage, x, was attached the 

 tube, n, for carrying the mirror, o, and the ring, p, for holding 

 the forceps, the condenser, and other things. The stage was 

 moved from side to side by the milled head, m', and up and 

 down by that at m. A condensing lens, q, was attached 

 by a moveable arm to the ring, p. This form of instrument 

 was adopted by the Tulleys (father and son), and by these 

 eminent opticians some of the first microscopists of the day 

 were supplied with it, amongst whom the names of Mr. 

 Lister, the late Mr. Loddiges, and Mr. Bowerbank re- 

 quire especial notice, as these gentlemen are intimately 

 associated with the rise of microscopic science in this 

 metropolis. 



While these experiments were in progress, Dr. Goring is 

 said* to have discovered that the structure of certain bodies 

 could be readily seen in some microscopes and not in others. 

 These bodies he named test objects, he then examined these 

 tests with the achromatic combination before noticed, and was 

 led to the discovery of the fact that " the penetrating power of 

 the microscope depends upon its angle of aperture." 



On the 30th of March, 1825, M. Chevalier presented to 

 the Society of Encouragement an achromatic lens of four 

 lines focus, two lines in diameter, and one line in thickness in 

 the centre: this lens was greatly superior to the one before 

 noticed, which had been made by him for M. SeUigues. 



In 1826, Professor Amici, who, from the year 1815 to 

 1824, had abandoned his experiments on the achromatic 

 * Microscopic Objects, p. 21. 



