146 



KNOWLEDGE. 



[July 2, 1894. 



steel tube sixty-four feet lonj?. supported at the centre by 

 the great declination axis. Messrs. Warner and Swasey 

 have done their work so well that we fear no serious 

 flexr.re, but a certain small amount must be expected. If 

 the aperture were trebled and the focal length doubled, a 

 single support would hardly be sufficient. 



With a reflector the conditions of the problem are quite 

 different, as the mirror comes at the lower end of the 

 tube. I have queried whether a slight modification 

 of Dr. Common's mounting for his five-foot reflector 

 would suffice for double the aperture and a much greater 

 increase in focal length. As the mounting is con- 

 structed, the polar axis is a great cylindrical steel vessel 

 so weighted as to float at the proper inclination in a tank 

 of water. The declination axis is near the lower end of 

 the short telescope tube, which is supported by it within a 

 fork projecting from the upper end of the polar a.Kis. As 

 the tube is very short and light, the mounting is a very 

 suitable one. If the fork were closed into such a form as 

 that shown in the drawing of Sir Howard Grubb's model, 

 and the upper end of the polar axis perhaps floated in a 

 second tank, a long tube supported near its middle point 

 might be carried fairly well, the range in declination being 

 about the same as in the case of the instrument described 

 in Knowledge. But as compared with the latter this 

 telescope would have, in common with most other instru- 

 ments, a number of serious disadvantages. 



In the first place, the tube would be so supported as to 

 show a marked eft'ect of flexure due to its own weight. In 

 Sir Howard Grubb's instrument the flexure of the lower 

 portion of the tube would be very small, and that of the 

 portion of the tube above the declination axis would also 

 be small on account of its comparatively short length. 

 There would, however, be a flexure produced by the 

 resistance of the water to the motion of the tube when 

 driven by the clock. It would be an important advantage 

 if the driving power could be applied directly to the lower 

 end of the tube, but on account of the motion in declination 

 this is probably impracticable. 



Again, the motion of the eye-end would be very great. 

 In the case of a great refractor like the Yerkes telescope, 

 the floor (seventy-five feet in diameter) of the observing 

 room must be arranged to rise and fall through a distance 

 of twenty-two feet. This seems unavoidable with a 

 refractor, but Sir Howard's plan reduces the motion of 

 the eye-end of an eight-foot reflector to so small a quantity 

 as to make it always accessible by very simple and in- 

 expensive means. For a ten-foot reflector of one 

 hundred and twenty feet focal length the motion of the 

 eye-end would necessarily be greater, but there would 

 probably be no great difficulty in following it on a movable 

 stage with stairway, or the observer might readily be 

 carried on the telescope tube itself. But bearing in mind 

 the purposes for which the instrument is designed — 

 photography and stellar spectroscopy — this difficulty could 

 easily be avoided. I would mount the spectroscope on the 

 end of the tube, with the slit at the focus of the great 

 mirror and the axis of the collimator coincident with the 

 axis of the reflector. 



The slit jaws should be polished, and inclined so as to 

 make a small angle with the focal plane. A reflecting 

 prism and telescope, supported near the centre of motion of 

 the reflector, would enable the observer to keep a star on 

 the slit. This device, which has been used by Dr. Huggins 

 for years, is much superior to any other method of main- 

 taining a star on the slit. For the second important use 

 of the reflector the spectroscope slit could be pushed up 

 out of the focal plane, to give place to a carrier for 

 photographic plates. A small mirror, attached to the outer 



edge of the carrier, and adjustable in position, would make 

 possible the observation of a star outside the field being 

 photographed, with the small telescope used for a similar 

 purpose with the spectroscope. Thus, in both cases the 

 observer could stand near the centre of motion, and keep a 

 star on the slit or cross-hairs for hours. It would be 

 necessary to reach the upper part of the tube only for the 

 purpose of inserting the plate-carrier, and for visual 

 observations. 



Another difficulty of ordinary mountings is the necessity 

 of providing a very large and expensive dome to cover 

 them. The dome fulfils a double purpose ; it protects the 

 instrument against the weather, and shields it from the 

 wind during observation. The second point is perhaps 

 of more importance than the first, for a large telescope 

 might be so constructed as to sufler little from the weather, 

 its more delicate parts being covered. As for the wind, a 

 wall like that shown in Sir Howard Grubb's drawings 

 would sufficiently protect an instrument three-fourths of 

 which is under water. If a dome were desired, a com- 

 paratively small one would suffice. The great saving of 

 expense on the dome and rising-floor is an important 

 argument in favour of this form of mounting. 



Of the possible disadvantages which Sir Howard Grubb 

 has enumerated and discussed, it seems to me that the 

 first is the only one to be regarded as at all-serious. Con- 

 sidering its size and the immense number of interesting 

 objects within its reach, the limited range of the reflector 

 is no very great objection. The friction of the liquid 

 would probably render the setting rather slow, but such a 

 telescope is not designed to swing quickly about from one 

 object to another. It could better be employed a whole 

 night on one or two fields. As to the currents created 

 in setting the instrument, they would certainly be of brief 

 duration, and may therefore be disregarded. But the diffi- 

 culty arising from the difi'erence in temperature between 

 the liquid and the air is a more troublesome matter. That 

 the definition would be affected, and, worse still, the mirror 

 dewed if the air were warmer than the liquid, must be 

 expected. The remedies proposed might serve to remove 

 these difficulties, but I am somewhat sceptical about the 

 advisability of warming a mirror at the back. It might be 

 well to employ a double system of pipes for heating and 

 cooling, so controlled by a thermostat as to keep the liquid 

 very nearly at the temperature of the outside air. I should 

 still retain the proposed double tube, with the constant 

 circulation of air ; but even if every such precaution were 

 taken, much trouble from dewing of the mirror might still 

 be feared. Heating the mirror at the back, if it could be 

 nutiKKjed so as not to interfere with the definition, would 

 probably be the most efi'ectual remedy for dewing. It is 

 hardly necessary to add that, in our northern latitudes, some 

 liquid of lower freezing-point than water would have to be 

 employed. 



INSECT SECRETIONS.-III. 



By E. A. Butler, B.A., B.So. 



{Continued from {laye 123.) 



OST people have probably noticed at some time or 

 other certain small, dark bodies shaped something 

 like mussel-shells, which are not unfrequently 

 to be seen adhering to the skins of oranges or 

 apples, or to the twigs and branches of apple and 

 pear trees, but few would suspect them to be of insect 

 origin. Yet such is the case, for in these " mussel-scales" 

 we have examples of the family Coecida, or scale-insects, 

 and it is in this family that we are to find our next 

 examples of insect secretions. Amongst the Coccida there 



M' 



