On a New Form of Ghost Micrometer. 11 



arrangement of mirrors such, that after a bisection is satisfactorily 

 made, images of the divisions of the micrometer heads and position 

 circles could be thrown into the field of the same eye-piece just 

 as the image of the reticule is here thrown in, and then the read- 

 ings of all the graduations could be taken without shifting the 

 eye from the eye -piece of the telescope. 



Lastly, as to the source of illumination. The illumination of 

 this micrometer is a much simpler matter than in any other form 

 of micrometer for bright line illumination, on account of the 

 small quantity of light required. Still, if some simpler plan could 

 be contrived that would enable a source of faint light to be placed 

 close to the micrometer, instead of having to throw it from a 

 distance, or conduct it down the tube by mirrors, &c., a great 

 advantage would be gained. We are, therefore, engaged in ex- 

 periments to investigate whether such illumination could not 

 be obtained from platinum wire or carbon rendered incandescent 

 by an electric current of low tension. These experiments are 

 mentioned to show that every branch of the subject is receiving 

 due attention. 



Note. — Since this paper was read we have had an opportunity of 

 examining the original papers of Dr. Lamont, Dr. Littrow, as well as 

 notes on a kindred mstrument devised by Professor Stampfer of Yiemia, 

 by himself and by Herr Reslhliber, Director of the Observatory of 

 Kremsmilnster in Bavaria. Dr. Lamont used a glass plate, set trans- 

 parently at a small angle with the optic axis of the telescope to intro- 

 duce the image of the lines into the field of the eye-piece. Professors 

 Littrow and Stampfer's instruments are essentially similar, the images of 

 the points or lines employed being formed by a small segmental lens 

 immediately outside the cone of rays from the O. G., the lines or points 

 tised being external to the telescope tube and at one side of it, and the 

 light from them directed to the image-forming lens by a small prism of 

 total reflection. The beam of light was thus unilatei'al, and the prin- 

 ciple of the mstrument identical with that figured on Plate III., fig. 5. 

 Professor Stampfer preferred bright points to bright lines, which 

 last were used by Dr. Littrow. 



