Illuminating, &c., with High Powers. By Rev. W. IS. Dallinger. 1 67 



its repetition was the difficulty. We had seen it — both of us — nay, 

 we had spent hours in the enjoyment of the profit of the hght 

 it gave. But manipulate as we would, we could not repeat it. So 

 we gave it up ; and then, by comparative accident, we secured it 

 again ! What could be the secret of its appearance ? It was not 

 the centering as usually employed, that was palpable; to all appear- 

 ances it was not wholly deptendent upon the angle of the mirror, 

 for that was moved to every conceivable angle scores of times, and 

 for hours together, on different occasions, without result. The 

 amount of difficulty in securing this illumination may easily be 

 tested. Some of the most accomplished microscopists, who have 

 seen it, or to whom it has been described, have failed entirely by 

 ordinary means to reproduce it. At length, however, we saw 

 clearly that the whole secret was, after the ordinary centering was 

 complete, dependent upon the position of the image of the flame 

 upon the prism or mirror. We invariably use the flame edgeways 

 to the instrument, and employ a broad wick, so as to get depth of 

 flame ; and the effect was always secured, although with great 

 difficulty, by minute alterations in the height, or lateral position of 

 the light, so that, as it appeared, a certain point in the image of the 

 flame was exadhj under the optical axis of the condenser. This 

 was so manifest to both of us, that Dr. Drysdale devised a small 

 piece of apparatus that would specially apply to our needs — but 

 only to them. We had to work always with the instrument per- 

 pendicular ; so he had a small plane silver speculum placed under 

 the condenser at an angle of 45'', but so narroiv that it was easy in 

 comparison to direct the image of the flame to the right place. But 

 this was nevertheless difficult of adjustment, and involved alteration 

 every time ; for even the different height of the wick, or the least 

 change in the position of the lamp, was fatal to the result. 



It was thus quite clear that what was needed was a lamp, with 

 delicate motions in all directions; and such a lamp I have devised, 

 with the most satisfactory results as to the accomplishment of the 

 special end in view — the central illumination already described. 



In Fig. 6 the lamp is figured so as to present its general 

 aspect. A is the reservoir and lamp ; B the chimney ; C the 

 bull's-eye condenser ; D is the pillar which carries the lamp and 

 condenser, separately, up and down, by means of the milled heads 

 E, F (which are better seen in Fig. 7) and the racks. The racks 

 are placed on either side of the pillar, back and front. The front 

 ones are indicated at G- and H, Fig. 6, and by means of these the 

 lamp, which is fastened by an arm to a piece of metal J which 

 works in grooves, is carried up and down by the milled head E. 

 At the back of the pillar is another pair of racks, seen at K, L, 

 Fig. 8, which by the milled head F carry up and down a piece of 

 metal M, to which the arm of the bull's-eye N is attached. By 



N 2 



