16 



Upon finding that, after carefully cleaning the object-glass and the lenses of the 

 eye-piece, the telescope is not as clear as when first received from the maker, then 

 the cause of it is generally a film between the lenses of the object-glass we take 

 for granted that the lenses are not scratched or otherwise impaired but, as a rule, 

 it takes several years (with careful use sometimes many years) before such a film 

 has sufficiently developed to impair the transparency of these lenses. But when- 

 ever it is found that a film has settled between them, then it is best, if the distance 

 is not too great, to send the, whole, instrument to its maker, and if this is not feasible, 

 then the telescope, at least, well and soft packed in a box, should be sent. 



Cemented Object-glasses. To prevent the settling of a film between the 

 lenses composing an object-glass, and to avoid disturbing reflections of light from 

 their inner surfaces, such films and reflections imparting to an object viewed 

 through a telescope the hazy appearance noticeable in high-powered telescopes, 

 we now, since 1889, cement these lenses together, so as to form one lens only. The 

 lenses so treated are more efficacious in many respects than when separated by 

 three thin pieces of tin foil, as has been the custom of nearly all instrument 

 makers up to date. 



The cement, however, needs some five or six months to harden, and until it has 

 hardened sufficiently, an exposure to a cold atmosphere causing a greater contrac- 

 tion of the metal cell than the glass, the lenses are very apt to warp, which may 

 lead to a distortion of an object, when viewed through such an objective. 



The proper treatment of an object-glass freshly cemented is to keep the instru- 

 ment, when not in use, in a room having a mean temperature of about 68 F., or 

 slightly above. The same treatment should be followed if it is found that the 

 image formed of an object is slightly distorted ; only in this case the temperature 

 in which it is kept over night should be raised to about 75 or 80 F. This treat- 

 ment applies only to normally mounted objectives. If they are too tightly fitted 

 the lenses cannot be restored to their original efficacy without being attended to by 

 a maker. 



Object-glasses that are cemented are very apt to show some specks, or, with ill 

 usage, cracks in the cement, but, unless the specks are very numerous, so as to 

 cover almost the whole area of the object-glass, the opacity caused by them does 

 not sensibly affect the efficacy of the telescope, and therefore need not disturb the 

 mind. Our experience is that the usefulness of an instrument is greatly enhanced 

 when these lenses are cemented together, and that a few specks that may appear 

 after an exposure from a sudden change from hot to a very cold atmosphere, or vice 

 versa, are a lesser evil, as compared with the ill effects produced by a film that in 

 time will settle between these lenses if separated by pieces of tin foil, or even when 

 brought in direct contact with each other, as such a film will have much the same 

 effect as a fog, in preventing vision. 



When, after carefully cleaning the lenses of a telescope, the object-glass of which has 

 its lenses separated by pieces of tin foil, it is found that the image is not as clear as 

 originally, it is a sure sign that there is a film between its lenses, and that it has been 

 exposed to a damp or impure atmosphere, either by injudicious use in the field, or by 

 being left too long a time in the packing box, in which it is protected by cushions of 

 paper or shavings, both of which attract moisture, or by storing it away in its box in 

 such an improper place as a basement or cellar. Such film being noticed, it will then 

 be well to send the object-glass, or much better, the telescope, or, best, if the distance is 

 not too great, the whole instrument, to the maker, in order that the lenses may be 

 cleaned by him, and, if deemed advisable, be cemented. The slight expense incurred 

 of a few dollars will be more than justified by the advantage gained. 



When the object-glass, or telescope is returned after the cleaning or cementing of 

 its lenses, the cross-wire, spirit level, and vertical arc adjustments of the instrument 

 will require a thorough verification before it should be used. In case the whole instru- 

 ment has been sent to the maker, these adjustments are attended to by him. If the ob- 

 ject-glass has been cemented, the telescope should be watched for a year to see that there 

 is no distortion of the image. If there is a distortion, it will indicate that the object- 

 glass has been too tightly fitted, of which fact we should be informed, as also whether 

 after cementing the object-glass the instrument retains its cross-wire adjustment the 

 same as before the cementing took place. If the cross-wire adjustments have to be 

 more frequently made than before the lenses were cemented, it indicates that the o! 

 glass is iMt ti-htly lined to its cell ; and if such is the case it should be sent to us to be 

 more tightly fitted, after a lapse of about ten or twelve months, when the cement will 

 have sufficiently hardened to allow of a tighter fit of the object-glass in its cell. 



In telescopes of very hL-h power it is of as great importance to keep the lenses of the 

 eye-piece free from grit and films as of the object-glass. Therefore, whenever the tele- 

 scope does not appear to be clear, the lenses of the eye-piece need most careful cleaning 

 tour weeks). The cleaning must be done by first wiping gently 

 with a clean piece of old linen barely moistened with alcohol ami then wiping dry, using 

 a cle.r i the linen on every surface of the lenses. (Please read the various 



articles on on pages 13, U,and 33, of our handbook and catalogue.) To re- 



move the eye-piece, unscrew the German-silver screw at the eye-end of the telescope. 

 Of course, after cleaning, every lens must be put back in its tube precisely as marked, 

 and then the outer U-anngs of the eye-piece in the main tube must be greased with 

 tallow before the German-silver screw is restored to its place. 



