22 4 



THE POPULAR SCIENCE MONTHLY. 



quired for a telescope to adjust itself to out-of-door temperature, 

 except in the summer time, and it is better to allow an hour or two 

 for such adjustment in cold weather. Any irregularity in the 

 shape of the rings which persists after the lenses have been ac- 

 curately adjusted and the telescope has 

 properly cooled may be ascribed to im- 

 perfections, such as veins or spots of 

 unequal density in the glass forming 

 the objective. 



The spherical aberration of an object 

 glass may be undercorrected or over- 

 corrected. In the former case the cen- 

 tral rings inside the focus will appear 



faint and the outer ones unduly strong, while outside the focus 

 the central rings will be too bright and the outer ones too feeble. 

 But if the aberration is overcorrected the central rings will be 

 overbright inside the focus and abnormally faint outside the 

 focus. 



Assuming that we have a telescope in which no obvious fault 

 is discernible, the next thing is to test its powers in actual work. 

 In what is to follow I shall endeavor to describe some of the prin- 

 cipal objects in the heavens from which the amateur observer 

 may expect to derive pleasure and instruction, and which may at 

 the same time serve as tests of the excellence of his telescope. 



FIG. 7. THE OuT-or-Focus RINGS. 

 1, Correct figure ; 2 and 3, 

 spherical aberration. 



FIG. 8. Two VIEWS OF MARS IN 1892. The smaller with a three and three eighths inch 

 telescope ; the larger with a nine inch. 



No one should be deterred or discouraged in the study of celestial 

 objects by the apparent insignificance of his means of observation. 

 The accompanying pictures of the planet Mars may serve as an 

 indication of the fact that a small telescope is frequently capable 

 of doing work that appears by no means contemptible when 

 placed side by side with that of the greater instruments of the 

 observatories. 



