C. GENERAL PHYSICS. 171 



who shows that of eight different substances the spectrum 

 of the lisrht of fluorescence extends some distance on the red 

 end side of the principal absorption band in the spectrum 

 of transmitted light ; so that the spectrum of fluorescent 

 substances can never contain rays which are more refrangi- 

 ble than those which are most readily absorbed by a very 

 dilute solution. This, although a very general rule, yet has 

 some decided exceptions. In some substances, under strong 

 illumination, the light of fluorescence does contain rays of 

 greater refrangibility than those most readily absorbed by 

 a dilute solution, and extends from the red end a little be- 

 yond the centre of the main absorption band. A number 

 of little known and interesting fluorescent solutions are 



cj 



quoted by Sorby in illustration of his remarks. Monthly 

 Micr. Journal, p. 161. 



THE ISOCHROXISM OF THE BALANCE SPRING. 



William D. Glasgow, in a short article in the Horological 

 Journal, on balance springs, states that the isochronism of 

 the balance spring of a watch is a subject bristling with 

 controversy. There are some who say that every spring 

 must be isochronized ; others that every length of spring 

 has its isochronous point of suspension ; others that mere 

 length has absolutely nothing to do with isochronism. Mr. 

 Glasgow holds that length has every thing to do with it, as 

 shown by his own experiments. Too short a spring, what- 

 ever may be its form, will make the short arcs of the bal- 

 ance's vibrations to be performed in a less time than the 

 long* arcs. Thus a spring with ten turns may be too short, 

 and will lose in the short arcs and ^ain in the Ion 2; arcs. A 

 spring of two turns will be too long, and will describe its 

 longer arcs in too short a period. The best length for a flat 

 spring is, he finds, fourteen turns ; but a flat spring, al- 

 though the most common, is also the worst form, as it does 

 not expand and contract properly. It will assist the action 

 in this spring if it is always a little small, as this gives 

 more freedom to the portion of the coil next to the stud. 

 The Breguet spring, although differing very little in form 

 from the flat spring, is essentially different in action and 

 principle, having perfect freedom to expand in a circle all 

 around. From twenty to twenty-five turns is, he finds, the 



