26 



OPTICAL INSTRUMENTS. 



certain sections of the crystal, when we 

 require only a very small separation of 

 the images, we may preserve, on both 

 sides, the natural surface of the cleav- 

 age. The other means of procuring a 

 double image proposed by Dr. Brewster, 

 we shall subjoin, as being well worthy 

 consideration for micrometical purposes. 

 1 . The double image may be produced 

 by a small bisected plane speculum, 

 placed between the eye-lens and the eye, 

 and one of the halves may be made to 

 move by a screw, not for the purpose of 

 bringing the images in contact, but in 

 order to vary the constant angle, ac- 

 cording as it is wanted, for large or 

 small discs. 2. The duplicature of the 

 image may be effected by bisecting the 

 eye-lens, or by placing a bisected lens 

 between the eye-lens and the eye. 3. 

 The two images may be formed by a 

 slightly-inclined face, ground upon a 

 highly-polished and parallel plate of 



fluor-spar, one" image being seen by half 

 of the pupil through the parallel plate, 

 and the other through the inclined face. 

 Fluor-spar is recommended because of 

 its producing a less dispersion under a 

 given angle than any other substance ; 

 and even this might be removed by the 

 ordinary means. 



(42.) The graphic telescope is em- 

 ployed for the delineation of objects 

 situated at any distance from the in- 

 strument, which may be represented of 

 any required size ; and is used for draw- 

 ing portraits, landscapes, and architec- 

 tural subjects. It was invented in 1811, 

 by Mr. C. Varley,who obtained a patent 

 for it. This instrument consists of an 

 astronomical telescope of low power, 

 placed between two plane reflecting 

 specula, with a particular construction 

 of the eye-piece to correct the distortion 

 of the image produced by the eye-glass. 

 Fig. 27, is a section of the instrument. 



Fig. 27. 



O is the object-glass, E the eye-glass, 

 and F F two meniscus field-glasses, 

 whose form and distance from the eye- 

 glass are so adjusted as to produce a flat 

 and enlarged field of view. The rays of 

 light proceeding from an object enter 

 the side of the tube, and impinge upon 

 the flat speculum s, placed at an angle 

 of 45 with the axis of the telescope, 

 as is shown in section at A ; the rays 

 will be reflected by this speculum along 

 the axis and through the object-glass O, 

 and are converged to a focus near the field 

 lens F, where an inverted image of the 

 object will be formed and may be received 

 by the eye-glass E ; lastly, they will 

 strike the other inclined flat reflector, and 

 be reflected up to the eye placed above 

 at a. When a piece of paper is placed 

 below the speculum r, the observer being 

 supposed to look down on the specu- 

 lum, keeping both eyes open, he will be 

 able to see an image of the object on the 

 paper when the instrument is adjusted 

 to its proper focus. This representation 



maybe traced with one or both eyes, and 

 the size of the image may be varied by 

 altering the distance of the paper from 

 the speculum, or by changing the mag- 

 nifying power of the instrument. If the 

 first speculum s be taken away, the 

 sides of the objects are reversed (as on 

 an engraved copper-plate,) the specu- 

 lum r forms the image erect, which was 

 inverted by the eye-glass E, and pre- 

 vents the telescope from intercepting a 

 view of the paper.* 



* The inventor of this instrument has employed it 

 very extensively in sketching from nature ; and in the 

 mountainous districts of Wales the views become of 

 great value on account of their accuracy ; he has also 

 made with this instrument some very correct views of 

 the seat of the late Lord Byron, at Newstead Abbey, 

 Nottinghamshire. In drawing shipping and boats it 

 is extremely valuable, as the numerous lines in the 

 details of these objects are not of a geometrical 

 figure. We have been informed that this instrument 

 was the one employed in making the panoramic 

 view of the metropolis from the top of St. Paul's, 

 for the exhibition of which a building in the Regent's 

 Park, called the Coliseum, has recently been erected. 



