216 M. Frauenhofer's Descriptioti of a new Micrometer. 



of the circular line (but where there are many circular lines, 

 one of them at least must have an advantageous position), it 

 might even in this respect be desirable to make as many cir- 

 cular lines as is advisable for other reasons. Should even in 

 some of the circular lines the ingress merely have been ob- 

 served, and the interval of time had been too short, in order 

 to observe the egress too, yet in that case the observation is 

 not lost ; because the circular lines are concentric, and their 

 distances are known. 



The circular lines are so strongly illuminated by the lamp, 

 that their light does not vanish, even if a large star approaches 

 them. In very weak comets, however, this strong illumina- 

 tion might endanger the exactness of the observation. With- 

 out lessening the flame of the lamp, the illumination of the 

 circular lines can be diminished, by putting into the small 

 tube, with which the lamp is annexed to the eye-tube (fig. 2), 

 a smaller one, which contains a diaphram. The oil-vessel a 

 of the lamp can in every position of the telescope assume, on 

 an average, a horizontal situation, partly because it can be 

 turned round the axis b of the cylinder, which forms the 

 lamp, partly because the lamp in the small tube, with which 

 it is annexed to the eye-tube, can also be turned. The flame 

 is, in all positions of the telescope, in the axis of the lamp. 

 The light of the lamp, next to the flame, falls upon a convex 

 glass, through which it is thrown upon the micrometer. The 

 oil-vessel can be lifted out of the lamp, by pushing back at 

 the screw t*. By the pushing back of this part an opening at 

 the top of the oil-vessel takes place, through which oil may 

 be added. 



The glass on which the lines are etched, is so placed that 

 the etched surface is turned towards the eye-glass. A re- 

 versed position of this glass produces a disadvantageous re- 

 flexion, and the circular lines are less advantageously illumi- 

 nated. The three different eye-glasses can be screwed to the 

 same frame in which the etched glass is fixed, so that the lat- 

 ter need not be changed or brought out of its position, while 

 the different magnifying powers are applied. Each eye-glass 

 has in the front towards the plane-glass a diaphram of such 

 an aperture that it takes in the necessary part of the field ; 

 and betwixt this and the first eye-lens still a second one, 

 through which a part of the light, coming from the lamp, 

 which does not strike upon the circular lines, is intercepted. 



One might perhaps suppose that, if the etched glass were 

 not perfectly plane, it might have a very detrimental influence 

 upon the observations; but we may only imagine what would 

 take place if this glass was in reality perceptibly concave or 



convex ; 



