32 



OPTICAL INSTRUMENTS. 



sions, the two limbs are furnished with 

 a Vernier's scale. Spirit levels and a 

 magnetic needle are usually attached to 

 the^instrument ; and from the purposes 

 to which it is applied, a telescope with 

 this mounting is called a Theodolite, 

 derived from" two Greek words ^a< 

 to see, and V- the way or distance, 

 that is, an instrument for seeing or de- 

 termining distances. The method by 

 which the distances and heights of re- 

 mote objects are ascertained is by mea- 

 suring the angles subtended by the ob- 

 ject, and computing trigonometrically 

 therefrom. 



The continental writers have much 

 exaggerated the powers and penetration 

 of the telescope ; indeed the time is 

 not far distant, when it was gravely as- 

 serted, that works of art had been re- 

 cognised in our satellite the moon. The 

 fallacy of this circumstance may be 

 easily shown to our readers by the fol- 

 lowing simple considerations. Let a 

 person direct the tubes of a telescope 

 (without the glasses) to any celestial 

 object, and there fix them ; he will soon 

 find that in a short space of time, the 

 object will have removed from before 

 the mouth of the tube. Now this mo- 

 tion of the celestial bodies, which is only 

 apparent, arises from the revolution of 

 our earth on her axis ; and the quan- 

 tity of this motion may be determined 

 with facility, thus : the earth is known 

 to revolve once about her axis in 24 

 hours, and as eveiy circle is supposed 

 to be divided into 360 equal parts or 

 degrees, the apparent time any celestial 

 body takes to describe one degree, will 

 be found by dividing the 24 hours by 

 360, which gives us 4 minutes as the 

 time an object would pass the mouth of 

 the tube if it only takes in one degree 

 of the heavens. 



Now, if we suppose the glasses to be 

 placed in the tubes, the magnfying power 

 of the instrument being 60, and we 

 direct it (as before) to an object, as the 

 moon, whose diameter is about half a 

 degree, the time of her passing or tran- 

 sit will be one minute, if the field of 

 view be, as in the ordinary telescopes, 

 about 30, which the moon would ex- 

 actly occupy. If the power of the tele- 

 scope be increased 1 times, the eye- 

 piece having the same angle of vision, 

 onlv iis part of the moon would be seen 

 at once, 100 being the square of 10, the 

 increased power of the instrument ; and 

 the time this portion of the moon would 



pass the telescope is 6 seconds. Again, 

 if we increase the power 10 times, so 

 that its linear amplification of an object 

 is 6000 times, only a T oioo part of the 

 moon's surface could be seen in the 

 field of view ; or the planet Saturn, whose 

 apparent diameter is 10 seconds, would 

 just fill it, and the time of their passing 

 the instrument would be only T 6 o of a 

 second.* 



Having thus shown the amazing velo- 

 city a planet passes the mouth of a tele- 

 scope with these high powers, we shall 

 next proceed to point out the apertures 

 and amplification necessary for observ- 

 ing some given measure on the surface 

 of the moon. First, we must determine 

 the angle every object must subtend to 

 the eye, in order to render it visible : 

 this is found on an average for different 

 sights to be one minute, that is, when 

 an object is removed from the eye about 

 3000 times its own diameter it will only 

 be just distinguishable. From this we can 

 now determine the extent of the smallest 

 part of the moon's surface discoverable 

 by the unassisted eye. Its real diameter 

 being 2100 miles, which divided by the 

 number of minutes which its apparent 

 diameter subtends, (viz. 30,) gives us 70 

 miles as the measure of the least distinct 

 spot seen by the naked eye ; therefore, 

 we know that, if a telescope magnifies 

 70 times, we can just discern a spot 

 one mile in diameter on the moon's 

 surface ; and to recognise any object 1 

 feet in diameter, we shall find by this 

 rule the magnifying power of the tele- 

 scope must be 37100 times, and the 

 diameter of an object-glass or metal 

 for such an instrument may be found 

 by the method described in (18) ; 

 which if we suppose a pencil of rays 

 ? V of an inch in diameter will admit 

 sufficient light to the eye, the diameter 

 of the speculum must be 62 feet, and 

 its focal distance 309 feet, when an eye- 

 glass of yUh of an inch is employed. 

 These calculations must convince the 

 reader of our inability for making such 

 observations ; for if the impossibility of 

 procuring such enormous instruments 

 were overcome, they would be so un- 

 wieldy as entirely to prevent our using 

 them. 



* It is necessary for us to state, that, for the ob- 

 servation of small stars and nebulae with reflecting 

 instruments this power is occasionally employed ; as 

 even under these circumstances they possess no sensi- 

 ble diameter, and a regular motion is communicated 

 to the telescope in the plane of the equator by means 

 of a clock or other mechanism. 



