M. Frauenhofer's Description of a ?iew Micrometer. 217 



convex ; and we shall find, that, if not thick and if it stands at 

 the focus of the object-glass, no detrimental effect can take 

 place. As the eye-piece is not achromatic, the stars become 

 somewhat coloured at the margin of the field ; however, they 

 do not change their form when a circular line cuts them, and 

 even small star's do not vanish at the moment of the transit 

 through the illuminated circular line. I have tried to make 

 an achromatic eye-glass, but hitherto I see no possibility of 

 hitting upon one which shall, at the same time that the mid- 

 dle circular lines are plainly seen, show also the outer ones 

 tolerably plain, without moving the eye-glass. As the eye- 

 glasses strongly magnify, every particle which adheres to the 

 cut glass is perceptibly seen, because it is illuminated by the 

 lamp. It is almost impossible to keep the cut glass totally 

 free from dust. Although the illuminated particles appear in 

 the field of view like stars, yet we soon accustom ourselves to 

 their presence, and they are no obstacle to the observation, 

 because they do not change their places, but real stars are al- 

 ways in motion, and can therefore be easily distinguished 

 from those dusty particles. That too much dust should not 

 be suffered to adhere to the glass, is a matter of course. 



As in all instruments, where seconds of time are of conse- 

 quence, a firm position is the main object, so likewise is this the 

 case here : and nothing ought to be neglected which is capable 

 of producing it. If the micrometer is to give such exact obser- 

 vations as it is capable of, then the place where it is to be 

 put up, must be carefully chosen, every possible draft of air 

 avoided, and the observer must remain, during the transit of 

 the stars, unalterably quiet, without touching the telescope 

 with his eye, which is almost superfluous to mention. For 

 this reason, it might be well if the observer did not note the 

 observations himself, because he must change his position for 

 that purpose. A considerable alteration of the instrument, 

 during the transit of the stars, could however be often detected 

 at the calculation of the observations. 



For determining the relative place of two very near stars, 

 for instance a double star, there might still remain a good deal 

 to be wished for in the described micrometer. In these stars 

 the interval of time from the transit of the one to the transit 

 of the other, is much too short, in order to be observed with 

 any certainty. For this purpose a micrometer, consisting of 

 straight parallel lines (fig. 3), whose distances from each other 

 are exactly known and which cut through one another at an 

 acute angle, which is exactly ascertained, might be very ad- 

 vantageous. The glass on which these lines are etched can 

 be so turned on the telescope, that the lines which run pa- 



Vol. 63. No. 311. March 1924. E e rallel 



