VALUE OF INSTRUMENTS OF RESEARCH. 35 



and subdivision, degrees of force that are far too feeble to affect 

 the nicest balance of the ordinary construction, even if it were 

 possible to bring them to act upon it; and strange as it may 

 seem, it has been in such a balance that the Earth itself has been 

 weighed, and that a basis has been thus afforded for the compu- 

 tation of the weights of the different Planets and even of the 

 Sun ; whilst in the opposite direction it is employed to furnish 

 those data in regard to the intensity of the electric and magnetic 

 forces, on which alone can any valid theory of their operation be 

 constructed. 



The galvanometer, again, in which the minutest Electric dis- 

 turbances are rendered sensible by the deflection of the magnetic 

 needle, has not only brought to light a vast class of most inte- 

 resting electric changes which were previously unsuspected (one 

 of the most remarkable of these being the existence of electric 

 currents in the nerves of living animals, first ascertained by M. 

 du Bois-Reymond), but has enabled those changes to be esti- 

 mated with a marvellous amount of exactness ; thus furnishing 

 to observations made by its means, a precision which is quite 

 unattainable in any other mode, and which is absolutely essential 

 to the establishment of any valid theory of electric action. And 

 this same instrument is scarcely less valuable, as serving, by a 

 particular modification of it, for the detection and estimation of 

 changes of Temperature far too minute to be measured by the 

 ordinary thermometer ; thus affording the requisite means of ex- 

 actness to observation, in a department of science to which at 

 first sight it appeared to have no relation. 



" What an important influence," says Sir John Herschel, "may 

 be exercised over the progress of a single branch of science, by 

 the invention of a ready and convenient mode of executing a 

 definite measurement, and the construction and common intro- 

 duction of an instrument adapted for it, cannot be better exem- 

 plified than by the instance of the reflecting goniometer ; this 

 simple, cheap, and portable little instrument has changed the 

 whole face of Mineralogy, and given it all the characters of one 

 of the exact sciences." 



Of all the instruments which have been ( yet applied to scien- 

 tific research, there is perhaps not one which has undergone such 

 important improvements within so brief a space of time, as the 

 Microscope has received during the second quarter of the present 

 century ; and there is certainly none whose use under its im- 

 proved form has been more largely or more rapidly productive of 

 most valuable results. As an optical instrument, the Microscope 

 is now at least as perfect as the Telescope ; for the 6-feet para- 

 bolic speculum of Lord Rosse's gigantic instrument, is not more 

 completely adapted to the Astronomical survey of the heavenly 

 bodies, than the achromatic combination of lenses so minute that 

 they can scarcely be themselves discerned by the unaided eye, is 

 to the scrutiny of the Physiologist into the mysteries of life 



