Oct. 8, 1874] 



NATURE 



469 



tion, and so double the width of the dark lines, but will 

 produce no new ones ; it will also reduce the brightness of 

 the spectrum to its original state. Now, when we were deal- 

 ing with sodium light, we at this stage of proceeding 

 halved the width of the sht, for the images of the slit had 

 been doubled without their brightness being reduced, so 

 we could halve them and bring them to their original size, 

 and so increase the distance of separation still more ; but 

 with a continuous spectrum, if we close the slit we shall, 

 it is true, only decrease the width of each image of the 

 slit and not their brightness, but we decrease their over- 

 lapping and so decrease the brilliancy of the whole spec- 

 trum, and this we cannot afford to do, as we have started 

 with as narrow a slit as possible, and consequently with 

 as small a brilliancy as possible consistent with showing 

 the dark lines. We have therefore by this alteration of 

 size of glasses doubled the width of dark lines originally 

 visible, but we are not able to more than double the sepa- 

 ration of any two images of the slit, as we did with the 

 sodium light images, by narrowing the sUt in addition to 

 increasing the distance of the centres, and therefore no 

 new lines are produced ; in fact, the result of our change 

 of arrangement has been the same as a simple magnifica- 

 tion of the spectrum without a decrease in brilliancy ; 

 and an increase of prismatic power is exactly similar in 

 effect, as we shall presently show, though it seems at first 

 untrue that increase of prismatic power will not increase 

 the number of dark lines visible. Let us now double the 

 number of prisms ; then the length of the spectrum will be 

 doubled, and the distance of the centre of the images of 

 the slit doubled, and therefore more dark lines may 

 appear in addition to the original ones being widened, 

 but the brilliancy of the spectrum has been halved, and in 

 order to brighten the spectrum to the original state the 

 width of the slit must be doubled, which exactly undoes all 

 that the e.xtra prisms have done in producing more lines ; 

 for the images will e.xpand and obliterate the newly- 

 formed lines ; the original dark lines will, however, after 

 the widening of the slit, be double their original width ; so 

 that, as we have just stated, the increase of prismatic 

 power will not make a greater number of dark lines 

 visible. If we illuminate the slit more intensely, we may 

 decrease the width of the slit and still retain our original 

 brightness, and so obtain a reduction in the mdth of the 

 images, and consequently a greater separation between 

 their edges, and therefore an increase in the number of 

 dark lines in addition to increase of width of those origi- 

 nally visible ; so that for the same kind of light the 

 number of dark lines depends on the intensity of the 

 illumination of the slit. 



In dealing with the spectrum of an intense light like 

 that of the sun, where there are a large number of lines, it 

 is necessary to use an instrument of high power, whether 

 in number or size of prisms, in order that the exceedingly 

 fine dark lines produced by a low power may be, as it 

 were, magnified without loss of light, which is, as we have 

 shown, the effect of an increase of prismatic power ; and 

 in order that these fine lines may become visible and 

 sufficiently separated to render their identity for measure- 

 ment or otherwise complete, so there may be an apparent 

 increase in the number of lines by the invisible ones 

 being rendered visible by magnification without loss of 

 brilliancy in the spectrum. 



But in dealing with light like that from a planet or the 

 moon, where the slit must be so wide that iufi lines are 

 visible, it can soon be tested in practice that the increase 

 of power does not increase the number of lines. In 

 examining the light of the moon or of a nebula, or 

 any object having an appreciable diameter, any increase 

 of telescopic power for the purpose of forming the 

 image on the slit will not increase the useful 

 brightness of the slit ; for, supposing a spectroscope be 

 working to its greatest advantage on a telescope, then, if 

 the diameter of the object-glass of the telescope be 



doubled, the angle it subtends at the slit will be doubled, 

 and the cone of rays on the collimator side of the slit 

 will have its base doubled, and therefore it cannot all pass 

 through the collimating lens ; in fact, all the rays newly 

 added by the increase of diameter of object-glass will be 

 wasted against the tube of the collimator, and if we try 

 to utilise these rays by increasing the size of collimating 

 lens or decreasing its focal length, we shall also have to 

 increase the power of the eyepiece to get all the rays 

 into the eye, and so reduce the brilliancy of the spectrum 

 to its original state. In the case of increasing the focal 

 length of a telescope as well as its aperture, the brightness 

 of the image on the slit is not increased, but only its size ; 

 so the spectroscope is unaffected. But in the case of 

 viewing the spectrum of a star, matters are altered, for the 

 image of the star does not increase in size by increasing the 

 focal length of the telescope together with its apertures ; 

 but its brilliancy is increased, and therefore greater pris- 

 matic power can be used without increase of width of slit, 

 and more dark lines seen ; so that for stellar spectro- 

 scopy an increase of telescopic apertures is a direct 

 advantage. From the foregoing remarks we gain that in 

 the construction of a spectroscope the eyepiece should be 

 of as long a focus as possible, so as just to cause all the 

 rays to enter the eye; all magnification beyond this 

 means loss of brilliancy, and if the spectrum appea-(% 

 insufficiently large an increase in size of the collimatin g 

 and telescope lenses, together with the prisms, or an 

 increase in the number of the prisms should be made, 

 until the spectrum appears sufficiently large to suit the 

 requirements of the observer. G. M. S. 



THE SUB-WEALDEN EXPLORATION 



'"pHE Secretary of the Sub-Wealden Exploration has 

 -•■ just issued his eighth quarterly report, in which he 

 states that but little progress has been made during the 

 last three months in consequence of the mability to pro- 

 cure lining tubes of the required size in sufficient quantity. 

 The increased favour in which the diamond boring system 

 is now held has caused a great demand for these tubes, 

 and they are specially manufactured by an eminent Bir- 

 mingham firm. The new pipes are required for the diffi- 

 cult process of enlarging and lining the bore-hole to the 

 diameter considered requisite before attempting to with- 

 draw the broken rods, &c. Mr. Willett says : — 



" The engineers have no doubt whatever of their ulti- 

 mate success, and as the extraction of the rods is not a 

 matter involving the expenditure of our funds, we can 

 only regret the loss of the long summer days, and take 

 comfort from the assurance that, 'after the enlargement 

 and lining is accomplished, there is a much better prospect 

 of obtaining the desired depth of 2,000 ft. than there was a 

 year ago that we should reach half the distance (1,000 ft.), 

 provided always that the )-equisitc funds be forllicoining.' " 



He is anxious to dispel what he terms " the delusion " 

 that no more money is required from the public in conse- 

 quence of a Government grant to the work having been 

 obtained. He states that the Chancellor of the Exchequer, 

 with laudable foresight and prudence, has promised to 

 assist on certain condition?, to do which — 



" I. We must spend 400/. in boring tubes, &c. 

 II. We must bore 100 ft., which will cost 200/. ; and 

 tlicii, and not till tlu-ii, 



111. We can draw 100/. from the Exchequer, and so 

 on, claiming 100/. for every 100 ft. actually 

 explored." 



The third and last year of the tenancy for carrying out 

 the work has been entered on, and therefore the necessity 

 of speedily resuming the operations is at once seen. The 

 financial position is cheering, the present balance being 

 594/. 7J-. gr/. The honorary secretary says ;— 



" We are greatly indebted to the Right Hon. the Chan- 



