STARS IN COMA BERENICES. 401 



cellent instruments will be found in Dr. Scheiner's recent work 

 "Die Photographic dcr Gestirnc," p. 148. I shall say only a 

 few words on the subject : The essential features of the machine 

 are a strong iron frame, to which are attached a circular mov- 

 able plate- holder, and two parallel fixed bridges, one bearing 

 three microscopes and the other a straight scale. The holder 

 is capable of rotation about its centre, and of motion in a direc- 

 tion perpendicular to the bridges. In this motion it is guided 

 by an accurately straight steel cylinder, which is long enough 

 to permit the entire plate to pass underneath the microscope 

 bridge. This latter bears, as already stated, three microscopes. 

 Two of them are permanently fixed to either end, and point at 

 a graduated circle on the circumference of the holder. They 

 contain comb-micrometers, and read to seconds of arc. The 

 third, or measuring microscope, is mounted on a straight guid- 

 ing-way, and has motion entirely across the plate, in a direction 

 perpendicular to the cylinder. It is evident that any point on 

 the plate may be brought into the field of view. At the left 

 hand end of the bridge is attached a lever arm, by means of 

 which the guiding-way together with the microscope may be 

 raised through a small angle. When in this position, the 

 microscope points at the scale. Readings are made by means 

 of a filar micrometer. This is so arranged, that two revolu- 

 tions of the screw carry the wires over one division of the scale, 

 i. e., over one millimeter. The head is divided into one hun- 

 dred parts, so that twothousandths of a millimeter can be read 

 by estimation. As the machine was originally designed for the 

 measurement of reseau plates, the microscope has two screws 

 at right angles to each other ; they are designated as the hori- 

 zontal and the vertical screw respectively. 



From the above description of the machine, the method of 

 measuring follows immediately. The microscope being pointed 

 at a star, the micrometer is read ; then by means of the lever 

 arm, it is made to point at the scale, and, without moving the 

 microscope itself, the screw is turned until the threads cover 

 the next lower division, and the head is again read. The dif- 

 ference of the two readings, added to the number of the line, 



(61) 



