OPTICAL INSTRUMENTS. 



29 



(47.) Tho brightness of any object 

 seen through a telescope, in comparison 

 with its brightness when seen by the 

 naked eye, may, in all cases be easily 

 found by the following formula. Let n 

 represent the natural distance of a visi- 

 ble object at which it can be distinctly 

 seen, and let d represent its distance 

 from the object-glass of the instrument. 

 Let m be the magnifying power of the 

 instrument, that is, let the visual angle 

 subtended at the eye by the object when 

 at the distance n, and viewed without 

 the instrument, be to the visual angle 

 produced by the instrument, as 1 to m. 

 Let a be the diameter of the object- 

 glass, and p be that of the pupil. Let 

 the instrument be so constructed that 

 no parts of the pencils are intercepted 

 for want of sufficient apertures of the 

 intermediate glasses. Lastly, let the 

 light lost in reflection or refraction be 

 neglected. 



The brightness of vision through the 

 instrument will be expressed by the 



fraction ( an \ the brightness of na- 



\mp d) 



tural vision being 1. But although this 

 fraction may exceed unity, the vision 

 through the instrument will not be 

 brighter than natural vision. For when 

 this is the case, the pupil does not re- 

 ceive all the light transmitted through 

 the instrument. 



In microscopes, n is the nearest limits 

 of distinct vision nearly eight inches ; 

 but a difference in this circumstance, 

 arising from a difference in the eye, 

 makes no change in the formula, be- 

 cause m changes in the same proportion 

 with n. 



In telescopes n and d may.be ac- 

 counted equal, and the formula be- 



comes * 



CHAPTER X. Stands for Telescopes 

 Method of making Grinding and Po- 

 lishing Specula and Lenses Method 



' of centering and adjusting Lenses. 



(48.) To describe the numerous va- 

 rieties of stands or supports for tele- 

 scopes would be both prolix and tri- 

 fling, as different artists generally adopt 

 such contrivances as they think most 

 likely to please the fancy of the pur- 

 chaser ; but the chief consideration in 

 a scientific point of view is, to obtain 

 a steady and immoveable stand free 



* Barlow. 



from vibration. To effect this, ' the 

 instrument should be supported at 

 both ends, to give steadiness, and to 

 prevent its being affected by the wind ; 

 for every vibration will be increased in 

 the sanis ratio as the amplification of 

 the instrument, and produce a tremu- 

 lous or dancing motion in the objects. 

 Thus a superior telescope badly sup- 

 ported may be rendered inferior to a 

 common one fixed on an immoveable 

 stand. The materials of which stands 

 are composed should be capable of 

 transmitting as little vibration as pos- 

 sible ; thus, the vibration of a frame of 

 cast iron in one piece, although per- 

 fectly steady, would be sufficient to de- 

 stroy distinct vision. Wooden stands 

 are preferable, provided firm diagonal 

 braces are used, so as to form immove- 

 able triangular frames. Where iron is 

 required for durability, plates of lead 

 should be screwed between each piece to 

 stop the vibrations. In reflecting tele- 

 scopes the difficulty of preventing vibra- 

 tions greatly impairs their value ; for the 

 metals, particularly the small one, are 

 easily set in vibration, and unless the arm 

 of the small metal is damped, the vision 

 is frequently indistinct. When a Grego- 

 rian telescope has been taken fronTits 

 stand, and placed on a lump of soft 

 clay, the distinctness has been such as 

 to enable a person to read a bill placed 

 at 900 feet ; while on the stand it could 

 be read only at the distance of 650 feet, 

 although no apparent tremor could be 

 discerned when on the stand. 



(49.) A good composition for the 

 specula of reflectors is one of the most 

 important desiderata in the making of 

 telescopes. The qualities most in re- 

 quest are, a sound uniform metal, free 

 from all microscopic pores ; not liable 

 to tarnish by absorption of moisture 

 from the atmosphere ; not so hard as to 

 be incapable of taking a good figure 

 and exquisite polish, or so soft as to 

 be easily scratched ; and possessing a 

 high reflective power. The various com- 

 positions employed for specula differ 

 more in the admixture of minor ingre- 

 dients than in their essential materials. 

 Copper and tin (bronze metal) are the 

 metals mostly employed, with small 

 quantities of arsenic, silver, and brass. 

 The proportions generally employed are, 

 copper 32 parts, grain tin 15, with the 

 addition of two parts of arsenic to ren- 

 der it more white and compact. The 

 Rev. Mr. Edwards, in a treatise annexed 



