287 



as required; a pressure of 151b. being sufficient to force it out about |in. 

 This apparatus was put under the microscope with a lin. objective focussed 



E 



upon the capillary tube, and a micrometer scale in the eye-piece ; the air 

 was then forced out of the tube, and then the mercury was allowed to run 

 back till it coincided with the zero point on the scale. It was then ready 

 for use, and when the terminal wires (a & a') were put into contact with 

 anything electrical, the mercury was displaced, and the amount of dis- 

 placement was read off upon the micrometer scale. The motion of the 

 mercury in the tube was very rapid, it just darted forward and returned to 

 its original position, but that was quite sufficient to enable the observer to 

 note the exact position to which it reached along the scale. The equiva- 

 lents of the markings on the scale could be determined by placing a 

 galvanometer in the circuit (F), and comparing its indications with those of 

 the mercury, and the amount of pressure could be readily ascertained by a 

 barometer tube. The troubles attending the use of an extremely delicate 

 galvanometer (such as alone would be of any value in measuring very small 

 currents) were well known to all who were in the habit of using such 

 instruments, one of the greatest being the length of time required to get a 

 proper indication in consequence of the time occupied by the needle in 

 swinging to and fro before coming to rest; but the little instrument he had 

 described instantly indicated the amount of electrical force and with a 

 degree of precision which could not fail to be appreciated by all whose 

 investigations required such aid. Another application of the microscope 

 was to make it indicate the energy of the action of light of various degrees 

 upon the retina. He had made this the subject of a communication to the 

 Royal Society,* and would endeavour briefly to convey the substance of it to 

 the meeting. He took some engravings and put them under the microscope, 

 and magnified the lines, and then carefully measured the relation between 

 the light and shade, and afterwards compared them with the appearance of 

 the same lines at a distance, and with various shaded surfaces and the 



* See Proceedings Royal Society, 1877. 



