174 OBSERVATIONS ON NEBULiE 



done in Plate V. Farther than this, of course, it would be useless to multiply 

 lines; and these express the rate of condensation as well, and in much the same 

 way as a series of points distributed at very small, equal intervals serve to de- 

 termine the curve in which they are situated. 



This method may be designated as a melliod by lines of equal brightness^ in 

 analogy with the terms "cotidal lines," — "lines of equal magnetic intensity," 

 &c., to which modes of expressing facts it is nearly allied. 



13. In the double nebula, (Plate V.) the southern member of which is the 

 very remarkable and well known nebula divided into three portions by dark 

 rifts, and having a triple star in the centre, and the northern member, one 

 hitherto overlooked, and surrounding the bright star in its neighbourhood, the 

 liiies of equal brightness enable us to recognise at once particulars such as these : 

 that the portion of the trifid marked A suddenly shades off, almost to darkness, 

 on the side towards the triple star, as is indicated by the closeness with which 

 the lines succeed each other, while the transition outward is very slow and 

 gradual ; that the three nuclei of the southern portion, running up to shade 5, 

 are perceptibly brighter than that of the northern, whose brightest portion is 

 only within the line 4; and others of a similar description. It is evident, at 

 once, that all such particulars concerning a nebula as are expressed, in the 

 younger Herschel's nomenclature, by the terms "gradually," "suddenly," and 

 " very suddenly — brighter in the middle," are indicated in this method by a 

 greater and greater proximity and crowding of the lines; while the distinction 

 between such terms as these, "suddenly brighter" and "suddenly much 

 brighter — in the middle," are marked by a greater number of these lines inter- 

 vening in the latter case than the former. It will be easy to conceive how all 

 other particulars concerning a nebula, except, perhaps, its resolvability, and 

 the like, can be at once embodied in the simplest form of diagram at the time 

 of observation. 



14. The half lines serve to show strongly suspected gradations of shade ; for 

 instance, within the space enclosed by the line \\, the nebula is suspected to 

 brighten up a little, dividing the rift a into two branches. Many minute par- 

 ticulars are perfectly and readily distinguished at a glance in Plate V., but far 

 less easily and definitely in Plate IV. And even Plate IV., as well as the other 

 shaded drawings, owe much of their minute accuracy, in the hands of the en- 

 graver, to drawings similar in design to Plate V. They are, in fact, copied 



