of the Total Eclipse, 157 



physical value will be equally great without it) and writing 

 materials, and should be prepared at any signal first to note 

 the time, and secondly, to write down the phenomenon. A 

 fourth should observe the general appearances, as seen with 

 the naked eye. If the party were more numerous, a good 

 sextant, or other double-image instrument, might be found 

 useful in the hands of one person. 



6. It is important that the dark glasses used for observing 

 the sun up to the moment of total obscuration be so mounted 

 that they can be slipped off in an instant. And it is desirable 

 that each telescope should be furnished with several dark 

 glasses, some shewing the sun with a red disc, some with a 

 white or greenish disc. These may be mounted, in a form 

 which admits of rapid change, in a sliding or a turning frame ; 

 or their cells may be fitted loosely with bayonet-notch. If 

 the observer is satisfied with the use of one colour or com- 

 bination for the dark glasses, no arrangement is more con- 

 venient than that of wedges of the coloured glass, achroma- 

 tised (as to dispersion) by wedges of colourless glass ; the 

 intensity is then changed gradually by merely sliding the 

 combination of glasses. It may also be desirable to possess 

 the power of altering the aperture of the telescope rapidly : 

 this perhaps may be done by attaching by hinges to the 

 object-glass cell one or more flat rings, which can be turned 

 off or on the object-glass by pulling a string at the eye end. 



7. It is desirable also that the observers should be pro- 

 vided with some instrument for the measure of radiant heat, 

 as a thermomultiplier (of a coarse kind) with galvanometer, 

 an actinometer, or a simple thermometer with rough black 

 bulb (whose indications will be more accurate if the bulb be 

 inclosed in a glass sphere from which the air is exhausted). 

 In the selection of the instrument, it must be borne in mind 

 that in Western Europe the sun will be high, and that the 

 season of the year is (generally speaking) favourable to ener- 

 getic radiation / and also that it is desirable that the obser- 

 vation with the selected instrument occupy as short time as 

 possible. For meteorological observations, a dry thermometer 

 and a thermometer with wet bulb (or other hygrometer) will 

 probably suffice. 



