The objections to the camera obseura arc, 1st, That it is too large to be carried about with con enience.
The camera lucida is as small and portable as can be wished.
2dly, In the former, all objects that are not situated near the centre of view, are more or less distorted.
In this, there is no distortion ; so that every line, even the most remote from the centre of view, is as straight as those through the centre.
idly, In that the field of view does not extend beyond or at most 35°, with distinctness.
But in the camera lucicla as much as 70° or 80• might be included in one view." It is obvious, that the preceding contrivance may be applied to a telescope, for the purpose of taking sketches of the different objects that may be contained within the field of view ; but as it is only a small portion of a land scape, or of any large object, that can be seen at once through a telescope, it \I OUld be desirable to have some contrivance by which the objects seen in different fields of view, and sketched upon the same piece of paper, might be all connected with each other into one land scape. This, however, can be done only to a certain ex tent, as will appear from Plate CIX. 7. Let AB be the direction of the telescope, which, when placed upon a suitable stand, can be moved round the axis 0 in a horizontal plane, 13 b 6'; B, the extremity of the eve tube at which the prism of the camera lucida is fixed; AIN, the paper, lying in a horizontal position; and a b, a' b', successive positions of the telescope in a plane pa rallel to MN. Let El" be the field of view of the teles cope, when seen on the paper by reflection from the prism; then the instrument must he so constructed, that when the telescope is in the position a b, and directed to the part of the landscape immediately adjacent to that which is contained in the field EF, the field of view FG, when seen by reflection from the prism, must be in con tact \I ith El'. When this happens, we have 13 b = C c, and the angle 13F b = EBI", the angle subtended by the field of view; but it is obvious, that when the telescope is moved from the position AB into the position a b, its angular motion round 0, viz. the angle BO b, is equal to the angle comprehended by the field of view, that is, to the angle 13FE; therefore, in the triangles OB b, BF b, we have the angles at 0 and F equal, and the side B b eorninon, and consequently the side OB is equal to thr side BC. From this it follows, that in order to have the successive fields of view El', I G, G11, all joined to each other, or, at their proper relative distances, the distance of the eye from the paper must he equal to its distance from the centre of motion 0, round which the telescope revolves. The telescope should therefore be placed
upon a stand so constructed, that the centre of motion 0 may be placed in different positions between the eye piece and the object-glass ; by which means the observer may vary the distance of the paper from his eye, accead ing as he wishes, to have his drawing on a large or a small scale. By the instrument when thus constructed, we are enabled to take a connected panoramic view of any horizontal zone of a landscape, whose breadth does not exceed the field of view of the telescope. The objects contained in the different fields of view, will be arranged in a circle whose diameter is equal to the distance of ti.c eye from the centre of motion.
This instrument is admirably fitted fur taking a cor rect outline of the visible horizon, with all the various indentations with which that line is generally broken Lv the intervention of valleys and mountains. Unless the horizon is extremely and unusually contracted, the field of view of a common telescope will contain a zone, which will easily comprehend every depression and elevation ; and even when the place of the observer is embosomed in an amphitheatre of mountains which rise around him with various elevations, the field of few may be enlarged by diminishing the magnifying power of the telescope. For this purpose, the micrometrical telescope, in clued by Dr Brewster, is particularly applicable, as the magni fying power can be increased or diminished, without changing any of the lenses ; and as the distance between the eye and the centre of motion 0, can be altered, evci, though the telescope is fixed to its stand.° The micro metrical telescope having also the properties of a com pound microscope, any long object which cannot be cot. tamed in the field of view, in the direction of its length, may be delineated in a similar manlier. This contrivance cannot be applied to the common compound microscope. as it has not a motion round an axis.
'lie camera lucida of Dr Wollaston migl,t be fitted up with a horizontal motion, and without the aid of a teles cope, so as to delineate one continued zone of a land scape ; but when the objects are small, or at a conside ru ble distance, a telescope becomes indispensably maces sary. See the Philosofihical Magazine, vol. XX \ ii. p. 343 ; Nicholson's Journal, vol. xvii. p. 1. vol. xxiii. p. :372, vol. xxiv. p. 146 ; and Brewster's Treatise on Philoso fihical Instruments, Edin. 1812, book i. p. I 1, book iii. p 133, and book \ i. (o)