15 



Care of Telescope Lenses. 



As dust and moisture, as well as perspiration from the hands, will settle on the 

 surface of the lenses of a telescope, it becomes necessary that they should be 

 cleaned at times. A neglect to keep the lenses free from any film, scratches, etc., 

 greatly impairs the clear sight through the telescope. To remove the dimness, 



E reduced by such a film, proceed thus : Brush each lens carefully with a camel's 

 air brush, wipe gently with a clean piece of chamois leather moistened with al- 

 cohol, and wipe dry using a clean part of the chamois skin on every portion of the 

 lens, to avoid grinding and scratching. When perfectly transparent brush again 

 to remove any fiber that may adhere to the lens. The tubes in which the lenses fit 

 should be brushed, and if damp should be dried; this done, restore each lens to 

 its original place as marked. To remove dampness in the main tube of the teles- 

 cope, take out the eye-piece, cover the open end with cloth and leave the instrument 

 in a dry room for some time. 



If an instrument has been exposed to a damp atmosphere, or water has pene- 

 trated the telescope, moisture may settle between the crown and flint glass of 

 which the object-glass is composed. If such is the case expose the instrument to 

 the sun for a few hours, but if in the winter, leave it in a warm room some distance 

 from the stove, the moisture will then generally evaporate. However, if not suc- 

 cessful, unscrew the object-glass from the telescope, and heat it slightly over a 

 stove or open fire. If a film settles between these glasses nothing can be done ex- 

 cept sending the instrument to the maker. The two glasses form one lens only 

 and must not be disturbed, as upon their relation to each other the definition and 

 achromaticity of the telescope depends. Much depends also on the stability, with 

 which these lenses are mounted in their cell, as any looseness between them or the 

 cell will affect the adjustment of line of collimation. Of course, if at any time the 

 object-glass has been unscrewed from the telescope, this latter adjustment must 

 again be verified before the instrument is used. 



Additional Instructions concerning the Care of 

 Telescope Lenses, etc. 



Ever since the introduction of the high power in the telescopes of geodetic in- 

 struments, now used by the best makers, complaints are frequently made of 

 the loss of light in such telescopes and of the hazy appearance of objects 

 viewed through thetn, the latter in particular when an instrument has seen service 

 in the field for some time. Now, while the loss of light is wholly due to the greater 

 power as compared with the low powers formerly in vogue, and to the use of erect- 

 ing eye-pieces (see page 33), the "haziness" is produced principally by films 

 of dirt, settled on or between the lenses of a telescope, and becomes even more 

 marked as more lenses are used in a telescope. 



Perhaps it is proper to say here, that when comparisons are made between low 

 and high-powered telescopes of geodetic instruments, other things being equal, the 

 first named, as a rule, will incite favor, because, as in spy-glasses, the image of an 

 object seen through them has a brilliancy never attained by telescopes of higher 

 power. But, whenever the results of stadia work, or fine levelling, as obtained with 

 the more powerful telescope, are compared with those obtained by a lower power, 

 it will be found that, though less brilliant, the defining power of a high-powered 

 telescope is superior to the other within the customary range of distances had in 

 the ordinary engineer's and surveyor's practice. 



On the other hand, owing to the less amount of light with high powers, it is 

 necessary that the tine qualities of the superior lenses required for them should be 

 preserved, and on this account a more frequent inspection and a more careful 

 treatment of them is needed than when lower powers are used, inasmuch as the 

 least impairment of these lenses by films, or dust, etc., will reduce the defining 

 power accordingly. A little extra care, as consequent upon the use of high-powered 

 lenses, is, therefore, imperative, but in so doing one is more than compensated by the 

 satisfaction of having a finer and more penetrating telescope. 



To prevent an untimely settling of a film on the lenses of a telescope, and par- 

 ticularly that apt to form on the inner surfaces of the lenses composing an object- 

 glass that has not been cemented together such film being so fatal in an object- 

 glass because it cannot ordinarily be reached and without disarranging the cross-wire 

 adjustments the treatment of an instrument should be strictly in accordance with 

 the instructions given under "Prevention better than Cure," page 21. Unless these 

 conditions are complied with, the greater efficacy of a telescope composed of supe- 

 rior lenses will be entirely lost. 



