OF ARTS AND SCIENCES. 161 



the study of the zodiacal light, we cannot expect to determine the true 

 position of the light on any occasion by the simple methods heretofore 

 in use. We must either discover exactly what an observer means by 

 the boundary, and to what extent this boundary will be displaced by 

 given changes of brightness, or we must resort to direct photometric 

 observations. Tiie last course will probably be preferable. In the 

 absence of instruments, observers must not content themselves with 

 the casual comparisons heretofore made, but must compare together 

 different portions of the light, and also specified portions of the light 

 and of the Milky Way. The Milky Way itself must be studied in a 

 similar manner to learn the corrections for absorption due to the 

 varying altitude of its separate parts. Observations of this kind, it is 

 to be apprehended, cannot be made to advantage except in situations 

 peculiarly favorable to uniform transparency of the atmosphere at all 

 azimuths, and remote from large towns, in order to avoid the disturb- 

 ing effect of artificial light. 



Perhaps some photometric apparatus may be contrived to make 

 these determinations more generally practicable. Argelander's pro- 

 posal (A. 157) to compare a portion of the zodiacal light seen with 

 one eye and a star seen out of focus witli the other, may at least fur- 

 nish a suggestion for a photometer of the required kind, if it should 

 prove inapplicable in its original form. Wolf's method (W. 32) of 

 tracing the limits of the nebulosity about the Pleiades may likewise 

 prove serviceable in observations of the zodiacal light. It consisted 

 in watching the fine threads of a reticule, which became visible against 

 the nebulosity, but disappeared when the telescope was directed to the 

 darker sky beyond. Suppose a telescope to be provided with a series 

 of suitable reticules, and also with some apparatus for diminishing the 

 aperture or otherwise darkening the field. One of the most promising 

 methods of accomplishing this object is that employed by Loewy 

 (L. 92). A diaphragm having a circular aperture is made to slide 

 along tlie tube of the telescope, and its position is recorded by means 

 of a scale attached to the outside of the tube. If the reticules to be 

 used are of successive degrees of fineness, so that each will be as vis- 

 ible with a small aperture as the next in order is with a large aperture 

 against the same background, an extensive range of comparisons would 

 be practicable. The same apparatus might be employed in comparing 

 the imnges of stars thrown out of focus to a known extent, and a stan- 

 dard of comparison for faint lights might thus be established. But 

 much experience would be needed to test the value of a photometer of 

 this description. 



VOL. XIX. (S. S. XI.) 11 



