Jan. 3, 1878] 



NATURE 



189 



found in ventilating the tubes, i.e. making it of some 

 open-work construction, in order that the air may pass 

 through and across and remove currents of differing tem- 



peratures. This difficulty is not felt with refractors; but, 

 curious to say, in the largest refractor at present in exist- 

 ence (the Washington 2(3-inch), Prof. Newcomb informs 



Latiice Tube of ilie I.Itibcurnc Ken<.clir. 



me that considerable inconvenience is felt sometimes from 

 the outside of the object-glass cooling down more quickly 

 in the evening than the inside, which produces a decided 

 effect on the spherical aberration, and injures temporarily 

 the otherwise fine definition. He consequently recom- 

 mends the use of lattice or ventilated tubes for very large 

 refractors. If this be found necessary, this advantage of 

 the refractor vanishes." 



But there is another nice point concerning this larger 

 aperture which has to be considered. 



We may set out with observing that the light-grasp- 

 ing power of the reflector varies as the square of the 

 aperture multiplied by a certain fraction representing the 

 proportion of the amount of reflected light to that of the 

 total incident rays. On the other hand the power of the 

 refractor varies as the square of the aperture multiplied by 

 a certain fraction representing the proportion of trans- 

 mitted light to that of the totat incident rays. Now in 

 the case of the reflector the reflecting power of each unit 

 of surface is constant whatever be the size of the mirror, 

 but in that of the refractor the ifansmittin" power de- 

 creases with the thickness of the "lass, rendered requisite 

 by increased size. Although for small apertures the trans- 

 mitting power of the refractor is greater than the reflecting 

 power of the reflector, still it is obvious that on increasing 

 the size a stage must be at last reached when the two 

 rivals become equal to each other. This limit has been 

 estimated by Dr. Robinson to be 35*435 inches, a size not 

 yet reached by our opticians by some ten inches, but 

 object-glasses are increasing inch by inch, and it would 

 be rash to say that this size cannot be reached within 

 perhaps the lifetime of our present workers. However 

 this may be we can say with safety that up to the present 

 limit of size produced, refractors have the advantage in 

 light-grasping power, and it is also a question whether 

 with increase of thickness in the glass there will not be 

 such an increase in the purity of material and polish as 

 to keep the loss by transmission at its present value. 

 Any one who has a Tully and a Cooke object-glass, by 

 placing them side by side on a clean sheet of paper, will be 

 able to see how our modern opticians have already reduced 

 the loss by transmission. 



The next point worthy of attention is the question 

 of permanence of optical qualities. Here the re- 

 fractor undoubtedly has the advantage. It is true 



that the flint glass of some object-glasses, chiefly those 

 produced in Germany, gets attacked by a sort of tarnish, 

 still that is not the case generally, while on the other 

 hand, metallic mirrors often become considerably dimmed 

 after a few months of use, the air of a town seeming to 

 be fatal to them, and although repolishing is not a 

 matter of any great difficulty in the hands of the 

 maker, still it is a serious drawback to be obliged 

 to return mirrors for this purpose. There are, how- 

 ever, some exceptions to this, for there are many small 

 mirrors in existence whose polish is good after many 

 years of continuous use, just as on the other hand there are 

 many object-glasses whose polish has suffered in a few 

 years, but these are exceptions to the rule. The same 

 remarks apply to the silvered glass reflectors, for although 

 the silvering of small mirrors is not a difficult process, the 

 matter becomes exceedingly difficult with large surfaces, 

 and indeed at present large discs of glass, say of four or 

 six feet diameter, can rarely be produced. If, however, a 

 process should be discovered of manufacturing these discs 

 satisfactorily and of silvering them, there are objections 

 to them on the grounds of the bad conductivity of glass, 

 whereby changes of temperature alter the curvature, and 

 there is also a great tendency for dew to be deposited on 

 the surface. 



With regard to the general suitability for observatory 

 work this depends upon the kind of work required, 

 whether for measuring positions, as in the case of the transit 

 instrument, where pemanency of mounting is of great 

 importance, or for physical astronomy, when a steady 

 image for a time only is required. For the first purpose the 

 refractor has decidedly the advantage, as the object-glass 

 can be fixed very nearly immovably in its cell, whereas 

 its rival must of necessity, at least with present appliances, 

 have a small, yet in comparison considerable, motion. 



The difficulty of mounting mirrors, even of large size, 

 has now been got over very perfectly. This difficulty 

 does not occur in the mounting of object-glasses of sizes 

 at present in use, but when we come to deal with lenses 

 of some thirty inches diameter, the present simple method 

 will in all probability be found insufficient, but we antici- 

 pate that one will be adopted which will allow the per- 

 manent position of the object-glass to be retained. 



J. Norman Lockyer 

 {To be continued.) 



OUR ASTRONOMICAL COLUMN 



The Comet of 1106. — In Mr. WiUiams's account of 

 the object observed by the Chinese in this year, and called 

 a comet by Ma Twan Lin, we find the following note : — 

 "This appears to have been a large meteor, as it 

 seems to have been seen for a short time only." 

 It is probable that the author had not compared Pingrd's 



description ot the motion of the comet, which was 

 certainly observed in Europe early in the year, or he 

 would have seen that in all likelihood, notwithstanding 

 Ma Twan Lin's account reads as if it referred to a tem- 

 porary phenomenon, the Chinese really observed the 

 bright comet recorded by the European historians. We 

 are told that in the fifth year of the epoch Tsung Ning, 

 on day Woo Seuh of the first moon (1106, Feb. lo) a 



