260 Dr. A. M. Mayer on a simple Device for projecting on a 

 pies employed in this section. For since 



dx _ V dT , {[ dT_p L 

 ds ~ fM dx 1 l ds ~ V 

 we have 



fy = ±( y W\ £/ y dT\_d_/ dx\ 

 dx dx\ ds J ds V dx J ds\dsj' 

 For this proof I am indebted to Professor Clerk Maxwell. 



XXXIII. On a simple Device for projecting on a Screen the Be- 

 flections of the Needles of a Galvanometer. By Alfred M. 

 Mayer, Ph.D., Professor of Physics in the Stevens Institute of 

 Technology, Hoboken, New Jersey, U.S.A.* 



THE instrumental problem of obtaining on a screen the de- 

 flections of a galvanometer-needle in magnified propor- 

 tions has occupied the thoughts of several physicists. The sub- 

 ject is evidently one of considerable importance. In delicate 

 researches it is often necessary that the body of the observer 

 should be removed from the instrument, while' at the same 

 time he must be able to observe the minute deflections of its 

 needles. In lectures before our college-classes many of the 

 most interesting and fundamental phenomena of radiant heat, 

 electricity, and magnetism are often either entirely omitted or 

 imperfectly presented, in default of an instrument which can be 

 constructed by any one at a small outlay of time and expense. 

 The problem, therefore, has not been deemed below the serious 

 attention of eminent investigators ; and although there are 

 some who consider such contrivances trivial, yet I imagine 

 they would think otherwise if they had the habit of continued 

 original investigation, or the proper ambition to address their 

 students in the very language of Nature by bringing them face 

 to face with those phenomena which form the sure foundations 

 of our scientific reasoning. 



The method invented by Poggendorff, of observing the de- 

 flections of the galvanometer by reflecting to a screen a beam of 

 light from a small mirror attached to the needles, has been used 

 for many years. Sir William Thomson and Professor Tyndall 

 have extensively used this method ; and it has the advantage of 

 giving to the reflected beam an angular motion the double of that 

 given to the mirror by the needles. More recently Dr. Tyndall 

 has devised an instrument on the principle of the megascope. 

 He throws a vertical beam from an electric lamp on to the 

 dial and needle of the galvanometer, and by means of a lens and 



* Communicated bv the Author. 



