OF ARTS AND SCIENCES. 233 



the interval between the rectangles and the focal length of the tele- 

 scope the light in magnitudes corresponding to one second, or A, may- 

 be deduced. Perhaps the best method with a small telescope is to 

 measure a large number of stars whose light has already been deter- 

 mined photometrically, and deduce A from them. 



The great advantage claimed for this form of wedge photometer is 

 the simplicity of its construction, of the method of observing, and of 

 the computations required to reduce the results. It may be easily 

 transported and inserted in the field of any telescope like a ring 

 micrometer. The time, if the observer is alone, may be taken by a 

 chronograph or stop-watch. Great accuracy is not needed, since if 

 ten seconds correspond to one magnitude, it will only be necessary to 

 observe the time to single seconds. The best method is to employ 

 an assistant to record and take the time from a chronometer or clock. 

 If the stars are observed in zones, the transits over the bar serve to 

 identify or locate them as well as to determine their light. A wedge 

 inserted in the field of a transit instrument will permit the determina- 

 tion of the light of each star observed without interfering with the 

 other portion of the observation. If the stars are all bright, time 

 may be saved by dispensing with the thin portion of the wedge. In 

 equatorial observations of asteroids the light may be measured photo- 

 metrically with little additional expenditure of time. Perhaps the 

 most useful application would be in the observation of zones. When 

 the stars are somewhat scattered it would often happen that their light 

 might be measured without any loss of time. By this instrument 

 another field of usefulness is opened for the form of horizontal tele- 

 scope advocated at a former meeting of this Academy (Proc. Amei-. 

 Acad. XVI. 364). Very perfect definition would not be required, 

 since it would afFect all the stars equally. To an amateur who would 

 regard the complexity of an instrument as a serious objection to it, 

 a means is now afforded of easily reducing his estimates of magni- 

 tude to an absolute system, and thus rendering them of real value. 



