15 EASY AND ACCURATE METHOD of 



fiom one furface, will appear fingle and diftindt ; whei'eas 

 the imdge reflected from both iurfaces, will, moft fre- 

 quently, appear double or indiftinct; arifrng from the 

 want of parallelifm between the furfaces. 



4. The image of the fun feen by a fmgle rcfledlion 

 from one furface of the glafs plane, and that feen by a, 

 double refledion from the fpecula of the inftrument, 

 will both appear nearly of the fame fliade or degree of 

 brightnefs ; and this will feldom be fo intenfe as to re- 

 quire any fcreen or coloured glafs to be interpofed between 

 the eye and the image. 



5. In making obfervations with this inilrument, whe- 

 ther for the purpofe of adjufting the glalfes, or for any 

 other purpofe, where great accuracy is required, it is 

 neceffary that the point on the index-fpeculum from 

 which the firfl refiedion is made, that on the horizon- 

 glafs from which the fecond rcfledion is made, and the 

 eye-hole through which the image is viewed, be all in a 

 plane, parallel to the plane of the inftrument. This will 

 be effeded by wrapping a piece of tape, or the like, round 

 the index fpeculum, leaving only a bare ftrip of about a 

 quarter of an inch broad, parallel to the plane of the in- 

 ftrument, and at the fame height above it as the eye-hole, 

 and tranfparent part of the horizon-glafs. 



6. The eye-hole is generally made too fmall. In 

 raeafuring angles on land, as in the practice offurveying, 

 (for which purpofe this inftrument is much preferable to 

 any other in common ufe,) in taking altitudes at fea, in 

 meafuring the angular diftance of the fun and moon, but 

 efpecially of the moon and a ftar ; the eye has geneially 

 need of all the light that can be admitted from the objed 

 feen by dired vifion. — The eye-hole therefore, fhould, 

 1 think, be nearly as large as the ordinary fize of the 

 pupil of the eye ; nor is any inaccuracy, in this cafe, to 

 be apprehended from the line of vifion not being parallel 

 to the plane of the inflxument j the eye being capable of 



placing 



