OP THE rNlTED STATES. 381 



able by tlie other parts ol" the instrument, as it serves to ve- 

 rify these, and also gives certainty to the observer by the 

 accuracy of the pointing. 1 liave always found, in terrestrial 

 as well as celestial observations, the largest power wjiicli tlic 

 telescope could bear the most advantageous for use. Great 

 light is not an essential requisite. The distinctness of the 

 image and its size are lar preferable, as in terrestrial objects 

 they help in pointing minutely, and in celestial oljscrvation 

 they increase the quantity of visible or a{)parent motion. 

 The same considerations show, that there is no gain in a dis- 

 proportionate aperture. Accuracy of the image is lost by it, 

 as may be easily tried by diminishing the apertures of large 

 telescopes by covers of dilVerent openings. 



In the course of my gcodcsical operations, I was oliliged 

 only towards winter to lay aside the greatest magnifier 

 of the two feet theodolite, of seventy-seven times, and take 

 the next, of about fifty times. But soon after, oI)servations 

 at some distance became entirely impracticable, so as to give 

 the desired degree of accuracy. 



These circumstances render it proper to choose the dia- 

 meters of instruments such as to allow with propriety and 

 ease the application of some of the telescopes which are ob- 

 tained in creatcr perfection ; and in this respect it is proper 

 to mention here some facts relating to this subject. 



'i'he telescope which is obtained the most perfect in its 

 kind is that of three feet and a half focal length, and about 

 three inches aperture. For instruments, the aperture ol' 

 about two inches and a half is preferred. Below that size, 

 the telescope of thirty to thirty-two inches, with the same 

 apertures, is obtained in great perfection. These two regu- 

 late therefore the size of the largest moveal)le instruments. 

 Alter this, the telescope of twenty-two inches, with about an 

 inch and three-quarters aperture, which is well suited for the 

 eigbtecninch circles, is very good foritssi/c. Thesmallersizes 

 are made in great numbers and of various qualities, where 

 almost no distinction can lie made in respect to size, except 



VOL. II. 3 D 



