SEXTANTS AND OTHER REFLECTING INSTRUMENTS. 65 
the other, an arrangement is made for placing the shades to be tested in such a 
way that they can be turned round. Having placed in this frame a shade for 
trial, I bring the two images of the sun seen in the telescope into contact, by 
means of the micrometer screws ; and if, on turning the shade before the object- 
glass, the contact remains perfect, then we know that its surfaces are parallel ; if, 
on the other hand, the contact is broken, we can, by turning the shade, find the 
point of nearest contact; and by turning the shade through 180°, we measure the 
amount of opening of the images, by means of the divided micrometer head. This 
gives double the error which would be caused by the shade when used in making 
a direct observation. But as all angles observed by reflecting instruments are 
double, or, in other words, the divisions on the limbs of reflecting instruments are 
equal only to half those of an instrument used for direct observation, the double 
error given by this method goes all to deteriorate the observation, as an unparallel 
shade affects the angle when reflected by twice the amount’of direct observation. 
Another important object is gained by this method of testing the shades. It being 
a very difficult matter to obtain these altogether perfect, I reject all where the 
error would amount to 10’, which is the usual reading of a sextant ; and when an 
error of a smaller amount does exist, I find, by turning the shade, the line in 
which the want of parallelism exists; and by placing this line at right angles to 
the plane of observation in a reflecting instrument, the angles observed by such 
an instrument are not affected even by the small error of the shade. 
VOL. XVI. PART I. R 
