OPTICAL INSTRUMENTS. 



25 



Substance! to be employed for Refr.ictiv- 



Kye-Lenses. Power. 



1 . Oil of Cassia 1 . 64 1 



2. Aniseed 1.601 



8. Cummin 1.508 



4. Cloves 1 . 535 



5. Sassafras 1.532 



6. Sweet Fennel Seed 1 . 506 



7. Spearmint .... 1.481 



8. Pimento 1.507 



9. Flint Glass ., .1.616 



CHAPTER IX. Dr. Brewster 's Telescope 

 for measuring Distances - Double 

 Image Telescope Graphic Telescope 

 Achromatic Eye-Pieces. 



(40.) A telescope for measuring the 

 distances of objects was invented by Dr. 

 Brewster, for which he obtained a pa- 

 tent in 1810. When describing the tele- 

 scope/g-*. 15 and 16, it was stated, that 

 as the object approached the instru- 

 ment, the eye-tube required to be drawn 

 out to adjust the instrument for distinct 

 vision : now the measure of the quantity 

 required to be drawn out, if registered 

 on the sliding tube by divisions, and by a 

 corresponding mark on the outer tube, 

 would determine the distances of the ob- 

 jects. The increase of the focal length 

 of the object-glass may be found by 

 dividing the square of the focal length 

 of the object-glass by the distance of 

 the object, minus the focus of the ob- 

 ject-lens : thus, if the focus of the ob- 

 ject-lens is 2 feet, and the distance of 

 the object 50 feet, the tube must be 



drawn out 



or 1 inch from the 



oO 2 



solar focus, to adjust for distinct vi- 

 sion of the object ; but the length of 

 these divisions would be in a decreasing 

 ratio, and the quantity is almost imper- 

 ceptible for great distances : thus, if two 

 objects be 100 feet apart, and the near- 

 est 200 feet from the observer, the dif- 

 ference of adjustment for these two ob- 

 jects would be only T fj(j of an inch. In the 

 patent construction this quantity may be 

 increased to almost any required length, 

 by means of two object-glasses; the 

 inner one is about of the focal length 

 of the principal lens, and being fixed to 

 the eye-tube slides along with the eye- 

 piece. This instrument has been found 

 useful at sea in determining the distance 

 of head-lands, and in war, when an 

 enemy's ship is gaining sail ; this is de- 

 termined by taking two observations, 



and noting the difference of the quan- 

 tity required to be drawn out or pushed 

 in to obtain a distinct vision ; and when 

 the time between each observation is 

 known, the rate of the vessels sailing 

 may be ascertained. 



(41.) Another instrument, nearly for 

 the same purpose as the one just de- 

 scribed, but possessing more accuracy 

 for measuring angles and distances, was 

 constructed by the Abbe Rochon. This 

 instrument is the ordinary telescope, 

 with a double refracting prism of rock- 

 crystal, placed before the eye-glass ; this 

 prism produces two images of an object. 

 These images may be made to approach 

 to or recede from one another by alter- 

 ing the distance of the prism from the 

 eye-glass; but the object is better 

 effected by a method proposed by Dr. 

 Brewster, in which the first glass of 

 the telescope, together with the prism, 

 which may be cemented to it, is made 

 to slide to or from the other glasses of 

 the eye- piece, and the quantity moved is 

 registered by divisions on the tube. 

 This variable eye-piece, which changes 

 the power of the telescope, is so ad- 

 justed, that the two images of the object 

 shall be in accurate contact ; when this 

 is done, if an observation is made to find 

 the angle, the index, by previous expe- 

 riment, will give it ; but when it is desi- 

 red to know the rate of motion of the 

 object, another observation must be 

 made at a determinate interval, and the 

 difference of adjustment required in the 

 two observations to bring the images in 

 contact will determine its distance, the 

 scale being previously ascertained by ex- 

 periment or calculation. The imperfect 

 crystallization of quartz, and the difficulty 

 of working it, led Dr. Brewster to adopt 

 the colourless topaz of New Holland to 

 form his double-refracting prisms, it 

 being much freer from veins and imper- 

 fections, besides possessing the advan- 

 tage of a lower dispersive power. In 



