10 



Herman Schultz, 



that the observations be executed with an instrument of sufficient optical 

 power and with an appropriate eye-piece, and that the method of observa- 

 tion employed or the micrometer be fit for the purpose. As regards the 

 necessary dimensions of the telescope it is of course impossible to offer 

 any general opinion. I know however from my own experience, that with 

 an instrument of the size of the Upsala refractor much may be done; but 

 I have reason to believe it desirable, that nebula-observations should not 

 be undertaken with a much smaller instrument, especially if an illuminated 

 micrometer be employed. I hardly ever found any difficulty in the choice 

 of an eye-piece, a magnifying power of about 200 times appearing to be 

 the most advantageous in almost all determinations of position, though in 

 some cases a power of 300 times could be used with advantage. As regards 

 the choice of the micrometer, as far as I can trust my judgement, the choice 

 would seem wholly to lie between the parallel-line-micrometer with illumi- 

 nated spider-lines on dark field and the heliometer. With respect on the 

 other hand to the ring-micrometer, I must confess that I can not understand 

 its being otherwise than a totally inappropriate instrument, as a part of the 

 object at the very moment of observation is always necessarily concealed by 

 the ring, and I can not in many cases imagine, how it is possible sharply 

 to point a nebula, without having at the same instant the whole of its cen- 

 tral portion before my eye. The ring-micrometer is moreover an instrument, 

 which must, more than any other, leave free room for an observer's idiosyn- 

 crasies , and thus almost unavoidably give rise to sensible personal discre- 

 pancies in the results obtained, which in all probability would have been 

 very trifling, had the observations been made with an instrument, which 

 does not give such latitude to subjective fancy. The fact, that relatively 

 very excellent results have been attained by the use of the ring-microme- 

 ter for the observation of nebulae, of course only proves, that a skilful 

 observer can effect much with an inferiour or even an unfitting instrument; 

 but it by no means follows from this, that the parallel-hair-micrometer is 

 not preferable, whenever circumstances admit of its application and we have 

 to determine the positions with the highest degree of precision, which the 

 nature of the objects and other circumstances permit. 



