WITH A FOURTEEN FEET REFLECTOR. 173 



identical with the nebula of Plate IV. The lines marked 5 were traced in the 

 telescope among the stars, and imagined to surround all those portions of the 

 nebula which are of uniform brightness, and brighter than any other part. 

 The first perceptible gradation or diminution of light is bounded by the lines 

 marked 4, and so on, successively, to the line ^, which represents the utmost 

 bounds of the visible nebula. All these lines were first traced carefully by the 

 eye, in their windings among the stars in the field, retraced by the pencil upon 

 a map of the stars at hand, and finally corrected by repeated and mutual com- 

 parison. 



11. If we suppose in Plate V. the faintest perceptible tint to be laid over all 

 the space included within the line ~, and upon that, another layer of shade 

 bounded by the line 1, such that the gradation shall be just perceptible, and so 

 go on, increasing in depth of shade, till the last tint laid on within the lines 5 

 shall represent the brightest portions of the nebula, we have at once a repre- 

 sentation of /i.* 1991, giving, in its fullest perfection, the original idea of the 

 observer, as formed with the object under his immediate and minute inspection. 

 The great errors which are likely to arise in drawing and shading such deli- 

 cate objects are done away. In the usual mode, a slight pressure of the pen- 

 cil, or even the inequalities of the paper, may give a different impression of the 

 particular features of the object from that which the observer intended; and 

 where the gradations of shade to be represented are so extremely delicate, it 

 cannot go through the process of engraving without still farther suffering in 

 accuracy. In the method here proposed these sources of error are annihilated, 

 for these lines can be drawn and corrected out of doors, with the native object 

 in view, and can be transferred to the engraved plate without appreciable 

 alteration. 



12. In strictness, we should further suppose that these tints shade off" into 

 each other at their boundary lines, or that the lines should be drawn at infi- 

 nitesimal intervals from each other, as exact theory must require, and con- 

 formity with the gradual decrease of light in the objects themselves. But, in 

 practice, it is only necessary to draw the lines so that the spaces between them 

 shall represent the least equal gradations of light visible to the eye, as has been 



* The reference to Sir J. F. W. Herschel's catalogues is here, and elsewhere, by means of the 

 small letter A, according to his own notation. So, also, Sh refers to " Herschel and South's Cata- 

 logue," 1824. 



VII. — 2 T 



