TRANSACTIONS OF SECTION A. 449 



It is much used in the States for surgical operations, its extreme portability 

 and control rendering it peculiarly useful in this direction. 



The platinum loop can he raised to any temperature, and kept at the same 

 simply by the action of the foot on the bellows, leaving both hands at liberty for 

 operating, there also being an entire absence of fumes or other disagreeable smells. 



A battery of four small cells will heat 9 inches of No. 16 platinum wire to 

 redness. 



4. A Diagonal Eyepiece, for certain Optical Experiments, 

 By Professor G. Forbes. 



A diagonal eyepiece usually consists of a piece of plane unsilvered glass in the 

 eyepiece, inclined at an angle of 45° to the axis of a telescope, so that light can be 

 sent through the object-glass, while the telescope can still be used for direct vision. 

 In experiments on the velocity of light, the light which has been thus sent out 

 is itself examined through the telescope after reflection, and it is required to have 

 this light as intense as possible. The diagonal glass must play two parts, which 

 are in a certain sense contradictory. It must reflect as much light as possible 

 through the telescope, and it must let as much as possible of the return light 

 pass through to the eye. But at the time of emission the plane glass really allows 

 only jLth of the whole light to pass through, and on its return j|ths are allowed 

 to pass. Hence, on the whole, only ~ a , or - 06, of the light is received. Fizeau 

 and Oornu used double plane mirrors of microscope glass, thus doubling the 

 reflection, and the latter found that he got '16 of the whole light. But this in- 

 creases the evil caused by defects in the transparency of the glass, the imperfection 

 of the polish, and the presence of dust, which, however insensible in ordinary cir- 

 cumstances, produce a luminous field, which is very troublesome when powerful 

 lights are used. To remedy this the author used a plane silver mirror, with a hole 

 in the centre of such a size as to permit half of the light falling on the object- 

 glass to pass through. In this way half of the light is sent out, and half of the 

 return light illuminates the eye ; so that we utilise "25 of the light, and we have 

 a perfectly dark field in the centre of the hole. This is theoretically four times as 

 powerful as an unsilvered glass reflector, and practically the darkness of field 

 makes it ten times as good. 



5. A Clock with Detached Train. By Professor G. Forbes. 



In the course of some experiments still progressing, the author has used a clock 

 to give electric signals every second. It was made by E. Dent and Co., and has a 

 train of only one wheel and one pinion. This clock only goes for one hour, but 

 serves the purpose for which it was made. To make it go for a longer time, it was 

 driven by a weight hung by a pulley on an endless chain in the usual way, and a 

 common 5s. Swiss alarum clock was attached, which was connected with the chain, 

 to wind it up continuously. This answers so well that a similar construction is sug- 

 gested as not only the cheapest but also the best form of a clock with escapement. 

 It consists of a pendulum and escapement with no train whatever, with an endless 

 chain or thread passing over a pulley on the axis of the scape-wheel, and also over 

 that of a secondary clock, hanging between them in a festoon which supports the 

 weight by a pulley. The secondary clock gives the hours and minutes, and the 

 clock without train shows the seconds. We thus have a clock without the errors 

 introduced by a train. It is a gravity escapement without the locking friction. 



6. An Instrument for Indicating and Measuring the Fire-damp in Mines* 



By Professor G. Forbes. 



The instrument exhibited consists of a resonator of variable dimensions, and a 

 timing-fork of definite pitch. The resonator is a metal tube 1 inch in diameter 



* 'Proceedings' R.S.E. 1878. 

 1878. G G ' 



