July 1, 1895.] 



KNOWLEDGE. 



155 



any fine day, and repeated as often as desired. Moreover, 

 the declination of the sun is unimportant. 



The principle involved is that if two circles move with 

 a uniform relative velocity, c, so as to make both external 

 and internal contacts, and if ^j is the interval of time 

 between external contacts, and t.^ the interval between 

 internal contacts, the product of the two radii 77 r is equal 



to the very simple expression — ^-i— ^ — '^-^• 



If now the two circles are the sun and the field of view 

 of a telescope at rest, the radius of the sun is given in the 

 Xauticiil Almanac, and the value of r may be computed by 

 dividing the radius of the sun by the time [t^), also given 

 in the Xautical Almanac, which the radius of the sun 

 occupies in passing the meridian. In fact, if /■ refers to 



the sun r = — , and we have for the radius of the telescope's 



field, /; = 



16 



In practising this method there would be some uncertainty 

 about the time of the first external contact, as the sun, 

 being out of sight the instant before, cannot be watched up 

 to contact in this case. From the symmetry of the motion, 

 however, this first point can be computed from the fact 

 that it must occur as much before the first internal contact 

 as the second internal contact occurs before the last 

 external contact. 



It is, indeed, very convenient not to take the first contact 

 into consideration at all, for much the easiest way to 

 ensure the motion giving the internal contacts is to get 

 the sun into the field, and then to displace the telescope 

 slightly in the direction of the sun's motion. Things will 

 then be in a position to give the first internal contact in a 

 few seconds, and the two other contacts wanted must 

 follow as a matter of course. Modified in this way the 

 equation for the diameter of the field, D, becomes : — 



D = 



t, «., 



and f. 



the result must be increased by 5-5 per 



D (in minutes of arc)= 



5&8 



where ^3 t^ are taken in seconds. 



It need scarcely be pointed out that Ring-micro- 

 meters can be measured in the same way. 



2 t„^ 



When fj IS the interval of time between the first internal 



contact and the last (now the only) external 



contact, 



is the interval of time between the second internal 



contact and the external contact, 



'o (given in the Katitical Ahiumac) is the time of the 



sun's radius crossing a meridian. 

 /■ (given in the Xautical Almanac) is the radius of 

 the sun. 



If «3 t^ are given in the same denomination as t^, well 

 tnd gotd; but t^ is given in the Xautiial Almanac in 

 sidereal interval, and it will very olten happen, at least 

 with amateurs, that t^ and t^ are measured in solar in- 

 terval. If so, 

 thousand. 



_ The fact that the motion of the &un is not entirely at 

 right angles to the meridian, would not, under the most 

 favourable conditions for doing so, produce a greater error 

 than one part in COO.OCO, and its further consideration 

 may therefore be neglected. On the other hand, for those 

 who are content with less rigid approximations to truth, 

 the equation may be simplified by taking average values 

 of r and t^, in which case we have, dispensing with the 

 Nautical Almanac — 



NOTICE. 



The publishers of Knowledge have the pleasure to 

 annoimce that Dr. Isaac Eoberts, F.E.S., has kindly 

 oft'ered to continue in Knowledge his exquisite selection of 

 Photographs of Stars, Star-Clusters, and Nebulse, and the 

 first photograph will appear very shortly. The series is 

 intended to be in continuation of Dr. Roberts' recently 

 published work, " A Selection of Photographs of Stars, 

 Star-Clusters, and Nebula," which has contributed 

 largely to the extension of the knowledge of astronomical 

 phenomena. 



DR. ROBERTS' PHOTOGRAPHS OF STAR- 

 CLUSTERS AND NEBULA. 



By E. Walter Maunder, F.R.A.S. 



DR. ROBERTS has won for himself so high and 

 well-deserved a reputation by his photographic 

 studies of star-clusters and nebula?, that I am 

 assured the above announcement will give the 

 greatest gratification to every reader of Know- 

 ledge. More, it will carry out in the completest way the 

 programme which the late Editor, Mr. Ranyard, especially 

 proposed to himself. Realizing how marvellous and 

 powerful a weapon of astronomical research photography 

 had become, and the yet wider influence which it promised 

 to have in the future, Mr. Ranyard desired to place the 

 very best results of astronomical photography in the hands 

 of his readers, and the last four or five volumes of 

 Knowledge are a suflicient monument to his energy and 

 success. 



There can be no doubt that in so doing he inaugurated 

 a new epoch in observational astronomy. Hitherto the 

 great hindrance to the extension of astronomical observa- 

 tion has been its cost, and the heavy demands which it 

 makes upon the time of those who take it up. To purchase 

 and set up a telescope of sufijcient power to deal with any 

 object except, say, Jupiter and the Sun and Moon, is beyond 

 the reach of all but a very few ; to have the strength and 

 leisure to devote night after night to using the telescope is 

 a rarer good fortune still. Hence the amateurs who adopt 

 the microscope for their hobby outnumber those who adopt 

 the telescope by ten times over and more. 



The rise of astronomical photography has altered all 

 this. We have now many large observatories devoting 

 much of their equipment to taking photographs— photo- 

 graphs for the complete discussion of which they possess 

 no adequate staff'. In so doing they have, as it were, 

 brought down this or that portion of the heavens within 

 our reach to handle as we like. The photograph once 

 taken is for many purposes as good as— for some purposes 

 even better than — the original object itself. 



Here, then, is a fruitful field for those who have neither 

 the means to erect a large telescope, nor the leisure to 

 use it. The examination, the measurement, and the 

 reduction of astronomical photographs may well be taken 

 up as a most useful and interesting work by many lovers 

 of astronomy.who would otherwise find themselves debarred 

 from any active share in the advancement of the science. 

 We may put the matter more strongly. It not only may 

 be so taken up ; it oiii/lu to be, and no doubt before very 

 long we shall see this new order of astronomical workers 

 begin to be formed. 



We may indeed say that a commencement has already 



