March 2 1, 1878] 



NATURE 



407 



holes, and then run the needle through each, so as to 

 make the holes clean and even. 



■ Take one of these cards and one of the wooden slips, 

 and put the card squarely on one of the wooien blocks 

 and place the slip over it, and tack them both down to 

 the block. This will give us the cards and blocks as 

 shown in. the picture. When each card is thus fastened 

 to a block, we shall have two blocks left. These we can 

 lay aside, as we shall need them in another experiment. 



Now light the lamp, and place one block on the table, 

 quite near the lamp. Look at the lamp carefully, and see 

 that the flame is just on a level with the hole in the card. 

 If it is too high or too low, place some books under it, or 

 put the lamp on a pile of books on a chair near the table. 

 Take a chair and sit at the opposite end of the table, and 

 place another card before you. Now look, through the 

 hole in this card, at the first card before the lamp. If 

 the table is level, you will see a tiny star or point of light 

 shining through the holes in the two cards. Without 

 moving the eye, draw the third card into line between the 

 others, and in a moment you will see the yellow star 

 shining through all three cards. 



Next take a piece of thread and stretch it against the 

 sides of the three cards, just as they stand, and immedi- 

 ately you see that they are exactly in line. The holes in 

 the cards we know are at the same distance from the edges 

 of the cards, and our experiment proves that the beam of 

 light that passed through all the holes must be straight, 



Fig 4. 



or we could not have seen it. The cards are in a straight 

 line, and the beam of light must also be straight. This 

 experiment, like the first, shows us that there is a law or 

 rule governing the movement of light, and that law is, 

 that light moves in straight lines. 



Move the lamp as near to the edge of the table as 

 possible, and then bring one of the cards close to the 

 lamp chimney. Then change your seat, and repeat this 

 experiment several times in different directions. Each 

 time you will see exactly the same thing, no matter in 

 what direction the light moves from the lamp. The lamp 

 may be moved from one side of the table to the other, 

 and in every direction we shall find the light moving in 

 exactly straight lines from the source of light. This is 

 true whether the source be the sun, a lamp, or a star. 

 One can walk all about the lamp and see it from every 

 side, and we can place our three cards in any direction, 

 north or south, up or down, east or west, or in any and 

 every direction, and every time it will give the same 

 result. 



Thus we have found out the law by which light moves, 

 viz., it moves in straight lines in all directions from the 

 source of light. 



Knowing this, you can readily think of a number of 

 things in which these laws are made useful. A farmer 

 planting an orchard, an astronomer fixing the positions 

 of stars, a sailor steering his ship by night, employs this 

 law: the first, to arrange his trees in straight lines ; the 



second, to measure out vast angles in the sky ; and the 

 third, to lay the courses of his ship in safety. Each em- 

 ploys these laws with certainty and safety, because they 

 are fixed and never change. 



OUR ASTRONOMICAL COLUMN' 



Double Stars.— Vol. xliiL of the Memoirs of the 

 Royal Astronomical Society contains two series of micro- 

 metrical measures of double stars. The first, by Mr. 

 Knott, includes measures taken near Cuckfield, Sussex, 

 between the years i860 and 1873, with a refractor by 

 Alvan Clark, having an aperture of 'j\ inches, one of the 

 instruments formerly in the possession of the Rev. W. R. 

 Dawes. Measures of most of the well-known binaries 

 will be found in this series, as also of a number of objects 

 not so frequently under observation. Amongst the latter 

 is the suspected variable, U Tauri, which has been 

 observed on several occasions since November, 1863 ; 

 D'Arrest first pointed out that this star, supposed to be 

 variable by Mr. Baxendell, is really double ; it is included 

 in Schonfeld's last catalogue of suspicious objects with 

 the query, " welche Componente veranderlich .'"' Mr. 

 Knott's observations throw no light on this point, as he 

 appears to have failed to notice any certain traces of 

 change. A note referring to a star near /3 Leonis deserves 

 attention. Smyth, in his Cycle of Celestial Objects, gives 

 a measure, or, as it should perhaps be termed, an estima- 

 tion of the position of a companion to this bright star, 

 which he calls an eighth magnitude, and dull red, position 

 114°, distance 298", At the epoch i864"38 there was no 

 star of such magnitude in this place, but Mr. Knott 

 measured one which by the method of limiting apertures 

 was found to be 1 1 ■6m., position ii5°"4, distance 303 "'5. 

 The inference, especially in presence of Smyth's judg- 

 ment of the colour of his companion, must be that we 

 have here a new variable star. The Durchmusterung has 

 nothing in this position. 



The other series of double star measures to which we have 

 referred emanates from the Temple Observatory, Rugby, 

 and forms the second catalogue issued by Mr. Wilson 

 and Mr. Seabrcke. The previous catalogue was printed 

 in the preceding volume of the Memoirs, and contains 

 some introductory remarks that are wanting in the present 

 one. The selection of objects and the instrumental 

 means appear to be the same ; the stars are found either 

 in the Dorpat Catalogue or in the Pulkowa Catalogue of 

 1850. Amongst them may be noted O.2. 298, the first 

 measures of which by the discoverer gave, for 1846*49, 

 position 1 83°-8, distance i"'i9, while the Temple Obstr- 

 vatory measures, 1 873*48, assign for the position 232", 

 with an estimated distance, o" 45, and the intermediate 

 measures by Baron Dembowski, in 1866, confirm the 

 change in angle and distance. A great change is re- 

 marked in 2 651 ; at the epoch i829'67 we have, position 

 101° 8, distance io"'82, whereas the Rugby measures give 

 for i875'i8, position 59°*3, distance i6""26. In this case 

 it is probable that the alteration is caused by proper 

 motion of one of the components : thus the measures 

 may be reconciled, if we suppose an annual motion of the 

 principal star of about o"'243 in the direction I7°"9. Of 

 32 Orionis it is remarked " not divided, perhaps binary." 

 and the angle for 1874-1 is i98°-5 ; between 1830 and 

 1853 the distance appears to have been about one second 

 without any decided change in the position, which by a 

 mean of Struve, Dawes, and Jacob was 203° 6 ; the star 

 seems to require further attention. Of 33 Pegasi, another 

 object measured at Rugby, Struve remarks " comes in 

 coelo prorsus quiescit," or in other words the change in 

 angle and distance noted between his measures in 1829 

 and 1851, is due to the proper motion of the principil 

 star, wjiich, according to Madler, amounts to 34 'o in the 

 century, in the direction 93°'5. Mr. Wilson's measures 

 of 0.5. 311 confirm the marked diminution of distance 



