NIGHT GLASSES. 2OI 



reasons, for general use. It is however still retained 

 for special occasions, as for example for reading off 

 signals, or examining a distant ship upon the horizon, 

 in clear weather; for these purposes the telescope 

 retains all its acknowledged pre-eminence; but for all 

 the ordinary requirements of constant daily service, 

 the binocular has practically entirely superseded the 

 telescope, both as a matter of convenience, and 

 because it is much the more generally useful instrument 

 of the two. 



This is especially so by night. In former days two 

 telescopes were in general use at sea, one for day, and 

 the other (a low-power glass with a larger eye-piece) 

 for use at night, when telescopes of high magnifying 

 power are of no service for the purpose that is of 

 clearing up dark objects so as to distinguish them 

 against a dark background. For this a good binocular 

 has been found to be by far the best of all night 

 glasses, and we believe nothing else is now employed 

 for such purposes on shipboard during the critical 

 operations of going in and out of ports by night. 



The importance of having lenses of as large a dia- 

 meter as possible, in binoculars for land and sea ser- 

 vice, has already been alluded to. These limits are 

 however irrevocably fixed, as they must in no case 

 exceed the distance, measured from centre to centre, 

 between the pupils of the owner's eyes. A glass 

 chosen to suit an individual therefore requires to be 

 accurately adjusted to this axis. 



Now the width of the human face of course varies 

 considerably in different people, but the average dis- 

 tance between the eye centres may be set down at 

 2f inches. A man -however who wishes to have things 



