Jan, 17, 1878] 



NATURE 



225 



THE MODERN TELESCOPE ^ 

 IV. 



THE next point to which Mr. Grubb refers is one to 

 which much interest attaches. It is now a long time ago 

 since Sir J. Herschel investigated the effects of diffel-ently 

 shaped apertures upon the images of stars. The figure 

 shows the effects they produce due to diffraction. 



An effect is also produced on the image if a round, or 

 triangular, or square patch is placed in the centre of the 

 object-glass. With the former the discs of the stars are 

 smaller, and the position of the diffraction rings is changed, 

 so that double stars can thus be measured, while in ordinary 

 circumstances the companion is hidden by one of the rings. 



Now in a reflector, unless, indeed, we use the front view, 

 the central patch is always present, and it is to this and 

 to the arm which supports it that the peculiar look of a 

 star in a reflector is due. Mr. Grubb does not hesitate to 

 ascribe to this the great difference of opinion that exists 

 as to the performance of the two classes of instruments, 

 and adds : — 



"A veteran and well-known worker with refractors 

 declared ' he never looked into a reflector without drawing 

 away his eye in disgust ; ' and workers with reflectors 

 cannot understand how the refractor workers can bear 

 that dreadful fringe of colour from the secondary spec- 



trum. The same applies to other matters. Newtonian 

 observers cannot understand how those who observe w ith 

 refractors or Cassegrain reflectors can bear to strain their 

 neck so in looking up through the tube ; while the refrac- 

 tor and Cassegram workers cannot understand how the 

 Newtonian workers will break their backs sitting or 

 standing bolt upright when they might be reclining com- 

 fortably on an easy chair as they do. After all, when this 

 comes to be investigated it resolves itself into but little 

 more than a question of to which telescope the observer 

 has been most accustomed. Each observer becomes in 

 time wedded to his own instrument ; he has done his 

 work with it, the credit of his discoveries is due to it, and 

 he naturally falls into the idea that no other can be as 

 good." 



We next come to those points in which the reflector is 

 stated to be superior to the refractor. These are absence 

 of secondary spectrum, superior applicability for physical 

 work, possibility of supporting mirrors irrespective of size, 

 and handiness of reflectors due to their short focal length, 

 and especially if the Cassegrain form be employed, VVith 

 regard to the first point, the experiments of Mr. Vernon 

 Harcourt and Prof. Stokes, in which they attempted to 

 produce two kinds of stars with rational or nearly rational 

 spectra, have failed to lead to any great hopes being 

 formed as to ultimate success, and the superior advantage 



Fig. 10. — Diffraction eftects produceil by apertures and sLopi of dilft 



.pes (Herbchiilj. 



of the reflector in the fact that there is no colour will 

 doubtless long remain. The superior applicability for 

 physical work is much more doubtful. At present we 

 know too little about reflection from metals and many 

 other points to lay down the law with certainty, and in 

 my own opinion Mr. Grubb's dictum is far too absolute 

 with regard to spectroscopic work. 



In another part of his valuable paper Mr. Grubb 

 measures the advantage of the reflector with regard to the 

 question of support ; he shows that an object-glass may 

 be supported by a central arm without loss of definition. 

 and even that the tube may be filled with compressed air. 

 He says : " The pressure required would be very small. 

 Suppose the objective to be forty inches aperture, and 

 600 lbs. weight, and that it was purposed to lift § of its 

 weight on the air cushion, a pressure of about g of a pound 

 to the square inch, or say g'g of atmosphere would suffice, 

 even when the telescope is at its maximum elevation." 



The remarks of Mr. Grubb on the practical diffi- 

 culties which supervene when increased aperture is 

 required, are best given in his own words : — 



" It may be said that the difficulty of manufacture is a 

 question for the instrument-maker alone, and not to be 

 discussed by those whose business it is to decide on the 



■ Continued from p. 189. 



form of instrument employed ; but it should be remem- 

 bered that any advance in the size of telescopes, refrac* 

 tors, or reflectors, over those at present in existence, 

 must be considered to be to a certain extent, an experi-' 

 ment, and the nature of the difficulties which will be 

 encountered can at present only be speculated upon, even 

 by the most experienced ; and therefore it behoves those 

 whose province it is to decide on the matter to inquire 

 diligently into the relative practicability of the various 

 forms of telescopes in order that they may not decide on 

 a form which might be, if ever accomplished, of great 

 usefulness, but which on trial would be found to be, in the 

 present state of art, impossible to manufacture. 



" With respect to refractors, the first great difficulty to 

 be met with is that of procuring suitable discs of glass. 

 Of our glass manufacturers only two firms seem to pos- 

 sess the secret of manipulation of optical glass, viz., 

 Messrs. Chance, Brothers and Company, of Birmingham, 

 and M. Feil, of Paris, a descendant of the celebrated 

 Guinand. Of these one at least speaks confidently of 

 producing discs up to one metre in diameter ; but when 

 I consider the difficulty which I know was experienced in 

 moulding the 27-inch discs for the Vienna objective I 

 cannot say that I feel the same confidence. These 40- 

 inch discs would require to be obtained in one single 



