and the temperature of t,he furnace was held constant hy the thermostatic regulator, urhich, a t the slightest cooling, opened to admit more gas, and vice i'ersa. Owing to the insulation of sand, considerahle lag existed between tlie temperature of the furnace and that of the disk. and consequently, slight changes in the temperat,ure of tbe former did not, affectthe disk. In order to maintain the desired tempcratiire in the disk, it vas nwcssary to operat,e the furnace at a higher temperature. The difference varied with each run, brit avcraged shout 100" C. at the annealing tempirsture, and padually decreased as the disk was cooled.
.&DVAWPACE OF I-SlSG h IAllV E X P I X S I O N
GI.
It is a f:& quite generally accepted that, where a telescope disk is to hc used as a reflector, the optical propertics of the glass used are of no pa.rticular importance, and tlnit from this standpoint it is, therefore, possible Lo employ any type of glass possessing good weat,liering qualities, such as, for example, the commonly used crown glass. However, tlie use of ordinary crown glass is n contrihut,ing factor to a well-knoun difficulty encountered in the use of reflecting telescopes, namely, the effect of texipernture changes. Astronomers using reflect,ing telescopes are familiar widh the effects caused by changes of temperature in parabolic mirrors, as, for example, when the mirror is exposed to radiation and becomes hcat.ed, the glass expands, and the shape of its surface changes. It is to minimize the effect of changes in temperature, convection currents, etc., that paraholic mirrors are usually protected by an insulating chamber. Furthermore, i t is an accepted experience that when these undesirable surface changes occur, the resultant effect on tile definition of the reflector is, on the average, toward under-correction, It has accordingly been suggested that this difficulty might be minimized provided the parabolic mirrors were made with a slight degree of over-correction.4 Obviously, however, it is preferable to eliminat,e entirely the cause of the under-correction, and this desideratum was kept in mind in determining tho composition of the glass to be used. The glass used for casting the disks mentioned herein was a borosilicate glass of the following cornposition:
......... ......... ......... As202
70.8 14.4 8.4
................ 0.2 100.0
Fro. 7--A 40.1~. TBLESCDP~ DISKS T A N ~ I N CON EneB (The ruler i s a 12-in. one1
A few observations on the general utilit,y of the regulating devices may be of intercst. In operating the electrical equip nient i t was merely necessary to check against the st,andard cell each day, roll up the record paper, and inspect the machine. The pen required fiUing every two or three days. The chief source of' trouble in this control is the possibility of an arc forming on one of the intermitt.ently operating contact points, which may not only c a n s erratic operation of the relays, but may fnsc the contacts and thereby impair tlie functions of the controller. I n one instance, an arc formed on one of the control disks at the point where t.he contact, slider changes from tlie "current on" side to the "cnrrent off" side. This carbonized the fiber of t.he imsulatin! disk, and short circuiting the two sides: caused the opening and closing coils to operate simult,aneously. Tlie gas regulator may he expected to give lit,tle trouble, although care must be exercised that tlie expansion tube he tightly fastened to the valve mechanism, for should it become loosened a drop in temperature will result. Also, t,he vdvc must be properly set before the run is commenced, so the desired range of temperature control is within the range of tlie instrument, for otherwise, as adjustments are subsequent,ly made to cool the furnace, a point @I1 he reached where further adjustment d l increase rather than diminish the t.emperature. This is due to the cam adjustment which has no minimum stop.
Its sclect,ion was based on the fact of its lower expansibility than that of ordinary crown glass, for it is elearly estahlishcd that a change of figure in a parabolic mirror during changes of temperature depends primarily on its expansibility. Ordinary crown glass has a coefficient of expansion of about, 0.ooO011 per ' C., hut, stnhle glasses can he made with expansion coefficients anywhere from 0.000003 up to and beyond the expansion of ordinary crown glass. The glass used liad a coefficient of expansion of ahout 0.000009. However, there is no great difficultyin making disks of glass with a still lower expa,nsion coefficient. It should nevertheless he noted that as the expansion coefficient is reduced, it becomes increasingly difficult to eliminate from the glass small bubbles, whicli seriously inkrfere with t,l,e formation of n satisfactory mirror surface. + I .S. Plsrketf,J . ROY. dilronom.
So:. Can., 11 (1020,. 1'33.
Centennial of Founding of IIobart College Hobart College, Geneva, N. Y., is to celebrate the one liundredth anniversary of its iounding on June 13, 1922. President Smith has deleEated Prof. W. I
