344 ON tIGHT. 



hidden and concealed infirmities they ate filled with, that, till 

 sawn out into boards, tlie juice is not to be known, and the 

 wisest may be taken in. 



III. 



Popular Statement of the leautiful experiments of Alalus, in 

 which he has developed a new property of light.* 



A solar ray is IT ^^ ^ ^°'^'" '"^X ^^ directed by means of a heliostat into the 

 thrown in the JI_J plane of the meridian, so that it shall form with the ho- 

 make'a^n'qirHe '''^°" ^" ^"S'e of ipo ]o'. Then fix a glass, not silvered, in 

 of IQ" 10' with ^uch a manner, that it shall reflect this ray vertically down- 

 [t7s therf re-" '^^^''^^- If below this glass a second glass be placed, exactly 

 fleeted by a parallel to the former, the latter will make, with the descending 

 ^^l^nrhJdr'-r'^' ^^^ ^" ^'"S^® of 35'' 25', and reflect it again parallel to its first 

 down -.—and direction. In this case, nothing remarkable is observfd. 

 then by ano- ^^^^ jf jj^jg second Jilass be turned, with its face directed to- 

 ther dear ° ,...,.. . , 



glass, so as to wards the east or west, but without altermg its inclination with 



pass with the respect to the vertical ray, it will no longer reflect a single par- 

 same inclina- . , ,. ,. , . , . ' . _ . , ^ 

 tion io the ho- '^^le o( light, neither from its first nor its second surface. 



rizon as before. And if with the same inclination preserved with regard to the 

 markal)^e'^hap- ^"^'"t'cal ray, the face be turned towards the south, it will begin 

 pens if the ray to reflect again the usual proportion of incidental light, 

 d*'^^'' '^^\'r "^" the intermediate positions, the reflection. will be more or 

 S. But if E. or less complete, accordingly as the reflected ray approaches more 

 W. the second ^.^. j^^^ ^j^^ j^^^^ ^f ,1^^ meridian, 

 glass reflects ^ 



no light at all. Under these circumstances, where the reflected ray comports 



itself so differently, it nevertheless constantly preserves the same 

 inclination wiih regard to the incidental ray. 



We, therefore, in this instance see, that a v.ertical rav of light 

 falling on a transparent body, acts in the same manner when its 

 reflecting face is turned towards the north orthe scuth, and in 

 adiffertnt manner when this face is turned towards the east or 

 the west ; although these faces still continue to forai, with the 

 vertical direction of this ray, an angle of 35° 25'. 



* See alsoPhllos, Journal, XXX, 95, 161. 192. 



Thest 



