502 



CATALOGUE.— PHrSICAL OPTICS. 



this point, the whole effect must be similar to that of a con- 

 vex lens. 

 Wollaston on horizontal refraction and on 



the dip. Ph. tr. 1803. 1. Repert. ii. III. 



419- Nich. VI.46. 

 Mudge. Ph. tr. 1800. 720. 



Looking over Sedgraoor, after a warm day, Glastonbury 

 tor was depressed 29' 50". 



DeUic on the apparent elevation of horizontal 



objects. Ph. M. XII. 148. 

 Horizontal refraction at Youghal. Beauford. 



Ph. M. XIII. 336. 

 Gruber on refraction near a warm surface. 



Gilb. III. 377, 439. 

 Woltmann on terrestrhal refraction. Gilb. 



III. 397- 

 Heim on an unusual refraction. Gilb. V. 



370. 

 Dangos on a horizontal refraction at Malta. 



Ph. M. XIV. 176. 

 Gorsse on mirage. Ann. Ch. XXXIX. 211. 

 Wrede on an atmospherical refraction by the 



walls of Berhn. Gilb. XI. 421. 

 Giovene on the fata morgana. Gilb. XII. 1; 



Gilb. XVII. 129. 



Brandes found the terrestrial refraction diminished when- 

 ever the air cooled suddenly. 



Castberg on the fata morgana at Reggio. 

 Gilb. XVII. 183. 



Thinks it a shadow. 



It may frequendy happen in a medium gradually vary- 

 ing, that a number of difTercnt rays of light may be inflect- 

 ed into angles equal to the angles of incidence, and in this 

 respect the effect resembles reflection rather more than re- 

 fraction. Y. 



Abstract of the Bakerian Lecture, by Dr. Wollastok, con- 

 sisting of observations on the quantity of horizontal re- 

 fraction, and the method of measuring (he dip at sea. 

 Journ. R. I. 



Dr. Wollaston notice* Mr. Monge's memoir on the 

 " mirage" observed in Egypt, as containing facts, which 

 fully agree with his own theory formerly published. From 

 his observations on the degree of refraction produced by the 

 air near the surface of the Thames, it appears that the va- 



riations derived from changes of temperature and moistnre 

 in the atmosphere, are by no means easily calculable ; but 

 that a practical correction may be dbtained, which, for 

 nautical uses, may supersede the necessity of such a calcu- 

 lation. Dr. Wollaston first observed an image of an oar 

 at a distance of about a mile, which was evidently caused 

 by refraction, and when he placed his eye near the water, 

 the lower part of distant objects was hidden, as if by a cur- 

 vature of the surface. This was at a time when a continu- 

 ation of hot weather had been succeeded by a colder day, 

 and the water was sensibly warmer than the atmosphere 

 above it. He afterwards procured a telescope, with a plans 

 speculum placed obliquely before its object glass, and pro- 

 vided with a micrometer, for measuring the angular depres- 

 sion of the image of a distant oar, or other oblique object ; 

 this was sometimes greatest when the object glass was 

 within an inch or two of the ^vater, and sometimes when at 

 the height of a foot or two. The greatest angle observed 

 was somewhat more than nine minutes, when the air was 

 at 50°, and the water at 63" ; in general the dryness of the 

 air lessened the effect, probably by producing evaporation, 

 but sometimes the refraction was considerable, notwith- 

 standing the air was dry. Dr. Wollaston has observed but 

 one instance which appeared to encourage the idea, that 

 the solution of water in the atmosphere may diminish it> 

 refractive power. 



In order to correct the error, to which nautical observa- 

 tions may be liable, firom the depression of the apparent 

 horizon, in consequence of such a refraction, or from its 

 elevation in contrary circumstances, and at the same time 

 to make a proper correction for the dip. Dr. Wollaston re- 

 commends, that the whole vertical angle between two op- 

 posite points of the horizon, be measured by the back ob- 

 servation, either before or after taking an altitude ; and 

 that half its excess above 180° be taken for the dip : or if 

 there be any doubt respecting the adjustment of the instru- 

 ment, that it be reversed, so as to measure the angle below 

 the horizon, and that one fourth of the diffisrence of the two 

 angles, thus determined, be taken as extremely near to the 

 true dip. It is indeed possible, that the refraction may be 

 somewhat different at different parts of the surface, but Dr. 

 Wollaston is of opinion that this can rarely happen, except 

 in the neighbourhood of land. Y. 



Irregular Refraction at various Altitudes. 



Kffractio solis inoccidui. 4. Stockh. 1695. 

 Engl. 8. London. 



Ace. Ph. tr. 1697. XIX. Lahire's remarks. 

 A. P. 1700.37. H. 112. 



