Prof. Olmsted on the Zodiacal Light. 311 



fore the latter part of September. I quote from my record for 

 September 25th, 1S35 : 



4 



^ It from 3 to 4J o'clock, a. bi. .Very faint. 

 Seen only by fixing the right eye on the region of Canis Major, and 

 carrying the left eye along the ecliptic. Covers Regulusand the cluster 

 in Cancer, and terminates a little south of Castor. 



The earh'est distinct view I have obtained of this body in the 

 evening sky, was on the 21st of November, 1837, when I have 

 the following record : 



Have constantly searched for the zodiacal light in the evening 

 since the 13lh inst. Imagined that that part of the milky way where 

 this light would cross it was more luminous than common, but the light is 

 ambiguous on account o^ the presence of Venus. But this evening ex- 

 areiined in company with three of my astronomical pupils, all distin- 

 guished for acuteness of vision. At 7 o'clock, Venus being near the 

 horizon and hid behind a cloud, \ve could severally define the bounda- 

 ries of the zodiacal light. By fixing the right eye on the milky way 

 near Altair, and the left eye near the head of Capricornus, we could 

 discern a pyramid less bright than the milky way, but still sufficiently 

 distinct to be sure of its presence. Its upper edge grazed « and ,«« 

 Capricorni and j? Aquarii, its vertex reaching to the right shoulder of 

 Aquarius. Light very feeble and diffuse, but the triangular space be- 

 tween it and the milky wav, embracing the Dolphin, perceptibly darker. 

 Elongation from the sun 90°. 



As a description of the zodiacal sufficient to guide the ob- 

 server I will offer the following: From the middle of September 

 until the latter part of November, he will confine his attention to 

 the morning sky. An hour and a half before daybreak (which is 

 at that season of the year, in our climate, about 4 o'clock,) he 

 will first discern a feeble, diffuse, and scarcely visible light, of a 

 pyramidal figure, extending from the horizon upward through 

 the zodiac to Gemini, covering Regulns and Presepe, and termin- 

 ating a little south of Castor. Near the horizon its material is 

 iJsually mixed up with the vapors that prevail there, so as to pre- 

 vent its forming a definite boundary at its base; but from an alti- 

 tude of a few degrees above the horizon, the light gradnally de- 

 clines until it fades into nonentity. Along the central part of the 

 pyramid the illumination is greater than'at the borders. From 

 the greater length and amplitude revealed to us by circumstances 

 peculiarly favorable for observation, we have reason to think that, 

 on ordinary occasions, we do not see the whole o( the body, but 

 that it really extends further than its visible boundaries, both m 

 [ength and breadth. If the observer continues to watch this 

 Dody from the middle of September onward through the month 

 of October to the middle of November, he will perceive that the 

 vertex or visible terminus moves along through the order of the 

 signs, and nearly at the same rate with the sun, appearing, on the 



