SCIENTIFIC NOTES. 137 



Professor Harkness's micrometer has, in the first place, a tripod 

 like an ordinary theodolite ; it has also the vertical axis and the 

 horizontal circle for measuring angles of position. On the top of 

 the vertical axis is a table on which the photo, is placed ; and it is 

 rotated under a microscope until the edge of the sun remains 

 bisected by the micrometer line during a whole revolution ; from 

 the table on which it rests a microscope projects for reading the 

 circle and getting angles of position. On the same tripod on which 

 this table works a very firm rectangular frame is placed ; it rises 

 about 5 inches above the photo, and carries two microscope stages 

 which move exactly at right angles to each other ; on each of these 

 are two microscopes, one of which reads the photograph, and the 

 other a glass scale placed by the side of it. Of course, with such 

 a perfect micrometer as this, the position of the planet on the sun 

 may be measured in several ways, and if thought desirable, mea- 

 sures may be taken as in Sir George Airy's. But I believe the 

 intention is, to measure first the angle of position, or the angular 

 distance of the centre of the planet from a vertical line passing 

 through the sun's centre, and then measure its linear distance from 

 that centre. 



It will be long before all this is done. Up to the time I left 

 England, August 14, they had not begun the actual work, and at 

 Washington they were in the same stage of progress, i.e. making- 

 ready preliminaries. In England and America they talk of from 

 two to three years before the work is done. 



In the transit circle, the most important of all astronomical 

 instruments, some remarkable changes are now being made. Most, 

 if not all, here present are aware that when graduated circles for 

 astronomical purposes first came into use, ard when the gradua- 

 tions had to be made by the repeated and painstaking application 

 of the compasses, which would, I fear, exhaust the patience of 

 a modern artist, quadrants of from 8 to 10 feet radius, and mural 

 circles of 8 to 10 feet diameter, were common ; and even when 

 the genius of the elder Simms succeeded in making a dividing 

 engine so perfect that it still stands unrivalled in the world, large 

 circles were still considered a necessity, and made of from 4 to 6 

 feet diameter. The result of long experience proves that it was 

 not so much in the graduation as in the optical power of the old 

 instruments the fault existed ; and now, when the optical power 

 is quadrupled, it is found that not only may the circles be reduced 

 but that actually better results can be obtained from circles only 

 2 feet in diameter ; and these, be it remembered, graduated with 

 the same engine that was used long ago. And so strongly, is 

 opinion setting against large circles that the best maker on the 

 Continent refuses to make a circle of more than 2 feet in diameter, 

 even when it is ordered ; and Simms, the best maker in England, 

 would rather make them of 2 feet than a larger size. 



