676 EEPOET — 1900. 



Tvpe IV. e. A single spot of irregular outline witli small companions not in a 

 train. 



Type V. An irregular group of larger gpote. 



As an example, the process of formation and life-history of a composite 

 disturbance which crossed the solar disc five times during the period May 14- 

 September 4, 1887, could be succinctly described as follows : 



I. II. b. I IV. d. IV. a. I IV. a. IV. d. IV. a. I IV. a. I. II, a, I I. 



6. On a Chea'p Form of Micrometer for determining Star Positions on 

 Photograjyhic Plates. By H. H. Turner, M.A., F.R.S., Savilian 

 Professor. 



The experience of those who have been worlilng at the Astrographie Chart 

 shows that for measuring star photographs a reseau is practically indispensable. 

 The rcseaux made by M. Gautier of Paris may be treated as sensibly correct for 

 nearly all purposes. The cost is between '21. and 5/., according to the number of 

 lines ruled ; and this initial expense is in any case necessary. But since a photo- 

 graphic copy is often as good as the original, this expense might be shared 

 between two or three workers, or borne by some society, which could distribute 

 copies for a few shillings each to its members. 



Given the reseau, the rest of the micrometer can be made at a very small 

 expense (say oO«. at most) with wood, glass, and paper. Of course, some conveni- 

 ence is sacrificed and a little accuracy lost. The micrometers in use at the Gape of 

 Good Hope cost 180/. apiece ; the duplex micrometer used at Greenwich about 

 100/. ; even the simple form used at Oxford costs 30Z. It is not to be supposed 

 that nothing is gained by such expenditure. But with a very simple form of 

 instrument such as that exhibited, which a man could make for himself or with a 

 little assistance from any carpenter, excellent work can he done. 



The chief part of the instrument is the microscope with scale in the eyepiece. 

 Most people have some old microscope which would do quite well, and in any case 

 a cheap one is all that is necessary. The scale in the eyepiece can be made 

 photographically, drawing the scale on a large sheet of cardboard and takino- a 

 miniature photograph of it. 



The plate-holder is merely a sheet of glass on which the plate can be easily 

 moved backwards and forwards. The screws required for slow motion and 

 clamping maj^ be ordinary electrical binding-screws or something similar ; the 

 counterpoises bags of shot ; the reflector a penny mirror, and so on. 



To measure photographs which have no reseau already impressed upon them, 

 a photographic copy of a reseau may be bound up in contact with the plate in the 

 manner of a lantern slide. Attention may, however, be directed to a method of 

 impressing the reseau on such plates which have been already developed and 

 fixed,' due to M. Bourget, of the Toulouse Observatory. 



TUESDAY, SEPTEMBER 11. 



The following Papers were read : — 



1. Comparison of Prominence dnd Corona Photographs taken at Santa Pola, 

 Spain, and Wadesboro, in North Carolina, during the Total Solar 

 Eclipse of May 28, 1900, By William J. S. Lockyer,M.A., Ph.D. 



This paper consists of a comparison of photographs taken at eclipse station?, 

 .5,000 miles distant from each other, namely, Santa Pola and WaHesboro. 



' Given in the Obseriiatiiry, May 19(50, p. 223 ; and in the Butlet-M Astrdnomintle. 

 March 1900. - • 



