234 Optics. — Hi/(hai/lics. — Effects of severe Cold. 



I OPTICS. 



Sir, — The following question has been pi'0])osed to many 

 Ijlwlosophical persons, without obtaining any satisfactory answer, 

 notwithstanding the importance of the inquiry in regard to the 

 construction of astronomical instruments ; I therefore beg leave 

 to ask vour readers, "What is the smallest angular space that is 

 perceptible to a sound unassisted eye, or to an eye assisted by 

 a telescope of a given power, aperture, &;c. ?" — In Dr. Gre- 

 gory's CEconoinv of Nature, vol. i. 238, I read, " Opticians smj 

 that the eye is not capable of perceiving any object which suij- 

 tends an angle of less than half a minute of a degree : the image 

 on the retina is in this Case less than y-^'-yo^dth part of an inch, and 

 the oltject itself, at six inches distance, less than the ■,-~-odth part 

 of an inch broad. All smaller objects are invisible." Lalande 

 ill his Astronomy says. Eight minutes, 8', are imperceptible on 

 a figure of a foot radius : " 8' sont insensibles meme sur une 

 figure d'un pied de rayon, telle quej'ai coutume de I'employer*," 

 in projecting the occultations of stars. The foot English =11 

 inches 3-1154 lines French. This is all that I have found in 

 answer to the above question, and this is not sufficient. I de- 

 sire to know also on what experiments or observations such as- 

 sertions are founded. 



To Mr. Tilloch. A. M. 



HYDRAULICS. 



General Andreossy, the late French ambassador at Constan- 

 tinople, has communicated to the First Class of the Institute va- 

 rious researches, accompanied by charts and drawings, respect- 

 ing the conduits which supply Constantinople with water. They 

 contain descriptions of the ancient and modern structures oh 

 principles not hitherto- known, and the application of which 

 promises to be highly advantageous in hydraulics, both with re- 

 spect to the simplicity of the contrivances and the oeconomizing 

 of labour. 



EFFECTS OF SEVERE COLD. 



M. Desgenettes, the celebrated French physician, who ac- 

 companied the unfortunate army which penetrated into Russia, 

 thus describes some of the phaenomena which occurred among 

 the troops who were exposed to the intense cold which was fatal 

 to so many thousands, during the retreat from Moscow. The 

 effects alluded to were perfectly new to M. Desgenettes, and will 

 doubtless be equally so to our medical readers : " I have heard 



* The rail'us of an arc being unity, the leni^th of a second = 

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