HARDWICKE' S SCIENCE-GOSSIP. 



273 



or the equatorial, and mechanism is so arranged that 

 when the observer presses a spring and thereby 

 "makes contact," a lever is depressed and a mark 

 made on the paper of the chronograph barrel. The 

 value of the spaces being known, the exact time of 

 the occurrence is thus recorded. 



Only a few observatories possess a heliometer. This 

 may be briefly described as an equatorial, the object- 

 glass of which is divided into two equal parts. Each 

 half can be moved in its own plane, and in each will 

 be seen a separate image of the object viewed. The 

 angular distance of two objects not very far apart can 

 in this manner be exactly determined. It need hardly 

 be said that the details of construction in such instru- 

 ments demand a very considerable amount of care. 



One branch of the science remains to be mentioned 

 — Celestial Photography, and with a brief account 

 of it our imaginary visits to the Observatory must 

 terminate. 



The first successful attempt to make a celestial 

 object record its own image on a sensitive plate was 

 made by J. W. Draper in 1840. Using a telescope 

 of five inches aperture he obtained some pictures of 

 the moon, one inch in diameter. Ten years later, 

 pictures of double that size were obtained with the 

 fifteen-inch refractor of the Harvard College Observa- 

 tory, with which instrument Whipple was the first, 

 in July 1850, to obtain a photographic image of a 

 star, the one selected being o Lyra?. Others soon 

 entered a field which promised such a rich harvest, 

 and two years afterwards de la Rue demonstrated the 

 possibility of using the camera attached to a reflector 

 with equal success. In 1857, twelve years after the 

 first solar daguerreotype — a poor one, it is true — had 

 been taken, de la Rue was commissioned by the 

 Royal Society to superintend the construction of an 

 instrument specially adapted for the purpose of photo- 

 graphing the sun. The photoheliograph, which was 

 the result of his labours, has served as the model on 

 the lines of which all instruments of that class have 

 since been made. The first eclipse of the sun to be 

 photographed was that which took place in i860, and 

 a satisfactory solution was thereby obtained to the 

 question, which had long exercised astronomers, 

 whether the red prominences really belonged to the 

 sun or not. Great strides have been made in the 

 photographic art and new processes have been dis- 

 covered, enabling operators to secure pictures with 

 greater rapidity and better results than formerly. 

 Rapidity is a very important point, for although 

 ordinarily good clockwork will impart a fairly equable 

 movement to a telescope, it is frequently found to be 

 not sufficiently accurate to enable one to keep the 

 image of a faint star or nebula on exactly the same 

 point of the plate for any considerable length of time. 

 Not merely have sun, moon, planets, stars and 

 nebulae been photographed, but also their spectra, 

 and thus permanent pictures have been obtained, 

 which can be measured and whose meaning can be 



interpreted at leisure. Another advantage of photo- 

 graphy is that the actinic rays impress on the plate 

 images of stars and nebulae quite invisible to the 

 naked eye. The most splendid specimens of stellar 

 photographs yet seen were obtained by Messrs. Henry, 

 of Paris. One of their plates discloses the existence 

 of nearly five thousand stars in a small patch of sky 

 in the constellation Cygnus, where only 170 had 

 previously been catalogued. Results so successful as 

 this and many similar ones have led astronomers to 

 the belief that the laborious process of observing and 

 cataloguing objects by the older method involves very 

 serious loss of time, and the principal observatories in 

 the world are now combining in the magnificent 

 project of forming a photographic celestial atlas on a 

 uniform scale. According to the plan adopted we 

 may hope that this gigantic design will be accom- 

 plished in the course of about ten years. When this 

 is completed, as it will undoubtedly be, our knowledge 

 of celestial space will be enormously extended, and 

 it is not too much to expect that great discoveries, 

 hitherto undreamt of, will be made. It will be for 

 another generation to form a second edition of this 

 celestial atlas, when, by comparing the two, changes 

 of position among the double and multiple stars, as 

 well as the nebulae, will at once betray themselves. 



BIRDS AND FIELD-MICE. 



THE remarks which Science-Gossip contained 

 in a recent number regarding the mischief 

 caused by the destruction of small birds, may well 

 apply to the indiscrimate killing of the larger ones. 

 I may perhaps mention a circumstance which came 

 under my notice some time ago. I had occasion to 

 make a sojourn of a few weeks at Wildbach, near 

 Frankfort. A large garden containing apple-trees, 

 vines, potatoes, etc., lay close to the house. The 

 ground of this garden was entirely riddled by field- 

 mice, and it was becoming a serious question whether 

 it would ever be productive owing to the swarming 

 of these animals. The surrounding fields also were 

 the haunts of thousands which wrought very con- 

 siderable damage to the. crops. The sight of the 

 garden just referred to was at once pitiable and 

 laughable ; pitiable because of the ragged and for- 

 lorn appearance caused by the raids oi the mice on 

 its products, and laughable, because at very short 

 intervals, two shining black eyes and a sharp nose 

 were to be seen protruding from holes in the earth, 

 evidently belonging to denizens who were ready to 

 show that discretion was the better part of valour, 

 or to make a bold dash for a plum or an apple as 

 soon as the coast was clear. When out, a footstep 

 was enough to make them rush back into their holes. 

 They were tolerably canny, however, for I perceived 

 by looking one day over my shoulder that as soon as 

 I had passed on they peeped out again, and scam- 



