436 SCIENCE PROGRESS 



star? We have to measure two co-ordinates, x and y, we 

 may repeat the measures of each : we may do this for 

 each of the two or three images which occur on the plate 

 (according to the plan already explained) and take the mean ; 

 and we may then turn the plate round into another position 

 and repeat the measures. (This last precaution will need no 

 explanation, to any one who has had experience of such 

 measurement : it detects and eliminates well-known personal 

 peculiarities in .the measurer.) It would therefore be easy to 

 adopt a plan of measurement which would involve recurring 

 to the same star 2x2x3x2 = 24 times, without real super- 

 fluity. Indeed, such a process would be definitely advisable 

 for a small piece of work wherein the utmost accuracy was 

 desired. But what we have to settle with regard to the 

 project before us is whether we can afford it. For comparison 

 let us take the minimum instead of the maximum advisable 

 programme : we can measure both co-ordinates of the star at 

 a single setting on a single image, and this would be the 

 actual minimum, but scarcely advisable — for there is nothing 

 to check a mistake. To check mistakes we must have at least 

 another measure ; and if we turn the plate round through 

 180° to make this, we shall at the same time eliminate the 

 personal errors referred to above. This, then, may be taken 

 as the minimum advisable programme ; it involves recurring 

 to the same star twice and twice only. It was adopted at 

 Oxford, and the work of measurement took even then a dozen 

 years : it will be seen how easily this might have been turned 

 into half a century or more. 



The second important question of detail concerns the 

 apparatus for measuring the fractions of a square. That 

 which first occurred to the astronomer was the micrometer 

 screw, with which he was already familiar in work at the 

 telescope ; at many observatories this type of instrument has 

 been adopted for use. A spider-line in the microscope is 

 set on the side of the resean square and the reading of the 

 micrometer screw noted : then the screw is turned until the 

 spider-line falls on the star-image and the reading noted 

 again : finally the screw is turned further until the spider-line 

 falls on the opposite side of the resean square and the reading 

 noted once more. From these three readings and a little 

 arithmetic the quantity required is deduced. This process 



