136 The Ottawa Naturalist. [Oct. 



Ont., the species has been captured by Mr. C. E. Grant, on June 

 10th. 



P. OBLiQUA. A single specimen of this species was collected 

 in 1900, at Bristol, Que., by Dr. Fletcher. 



P. Helata. The only Canadian record we have for this 

 insect is a single specimen taken at Ottawa on June 20th, 1907. 

 by Mr. J. W. Baldwin. 



METEOROLOGICAL OPTICS. 



By Otto Klotz, LL.D, F.R.A.S. 



As we were returning one evening from one of our delightful 

 afternoon natural history excursions, the bright disk of the moon 

 rose slowly from the eastern horizon and soon emerged as a huge 

 platter, arresting the attention and calling forth remarks from 

 every one. 



Probably no illusion in the heavens is so apparent as the 

 increased size of the full moon when rising. Every one knows 

 that the disk of the full moon when seen on the horizon appears 

 very much larger than about six hours later when it is in the 

 south and high up in the heavens. One might infer that the 

 moon is a great deal nearer to us when rising than when seen 

 high up in the sky. As a matter of fact, the reverse is the case, 

 for when the moon is above us it is nearer by the radius of the 

 earth, say about 4,000 miles, or 1-60 of its average distance. If 

 there were any question about the delusion, it is very easily 

 settled by turning an instrument onto the satellite and measur- 

 ing its diameter, when of course it would be found that the 

 diameter was practically the same in the two positions. Quite 

 a different phenomenon is the flattening of the disk of the moon 

 when seen in the horizon, for this would be confirmed by the same 

 instrument that we used for measuring the horizontal diameter. 

 The explanation of the flattening lies in the fact that the nearer 

 we approach the horizon the more the rays are bent or refracted, 

 so that the lower edge of the moon looks relatively higher than 

 the upper edge, i.e., the lower edge is thrown up more than is 

 the upper one, so that the moon looks broader than it is deep, 

 in short its figure is elliptical. But the discrepancy in the ex- 

 aggerated size of the moon when rising is not due to the refrac- 

 tion of the rays of light. We may state at the outset that the 

 illusion is a physiological phenomenon. 



We are accustomed to speak of the sky as the celestial vault, 



