232 CARNEGIE INSTITUTION OF WASHINGTON. 



Of M 8, 7 pairs of plates were made with exposures of from 3"" to 15™, 

 the seeing ranging from 3 to 5 on a scale of 10. N. G. C. 7009 was 

 photographed on a single pair of Seed 23 plates with exposures of 20^, 

 1", 3"", and 9"°. For examination, corresponding pairs of negatives 

 were mounted together on one of the measuring-machines v/hich had 

 been supplied with an additional microscope, so that the observer might 

 look quickly from one plate to the other in comparing the details photo- 

 graphed with the two instruments. The magnification was adjusted 

 to bring the image of the 60-inch plate to the same scale as that of the 

 100-inch. 



Extended masses of nebulosity show the same density, as was to be 

 expected, since the two instruments are of the same focal-ratio. But in 

 resolution, a glance is sufficient to show the superiority of the 100-inch 

 over the 60-inch, even in the case of M8, photographed at a zenith 

 distance of 60° with seeing as low as 3. 



The results for the Hooker telescope in the case of N. G. C. 7009, 

 photographed at zenith distance 46°, seeing 6, are very much the 

 superior of the two. Fine details are clearly shown which can not be 

 traced on the 60-inch negative, or at most are only suggested by irregu- 

 lar and undefined masses of silver grains. It is noteworthy, too, that 

 small details approximating star-images in size, such as the ansse, are 

 stronger on the 100-inch negative than on the 60-inch, although the 

 relatively large central part of the nebula is strictly comparable in den- 

 sity on the two negatives. 



Greater resolution is of course to be expected from the larger instru- 

 ment, because of the increase of scale in the ratio of 10 to 6, provided 

 the linear increase in the excursions of the optical image do not nullify 

 the advantage thus derived. These tests show conclusively that there 

 is a decided gain in resolution with seeing as low as 3 on a scale of 10. 



PHOTOGRAPHS OF NEBULA AND CLUSTERS. 



The following photographs of nebulae and clusters were made by Mr. 

 Pease with the 60-inch telescope for classification and study: 



N. G. C. 2146. Abnormal spiral. N. G. C. 6595. Two stars involved in nehu- 

 N. G. C. 3310. Spiral. losity. 



N. G. C. 5595, 5597. Spirals. N. G. C. 6643. Spiral. 



N. G. C. 6053. A group of small nebulaj N. G. C. 6906. Spiral. 



and nebulous stars. N. G. C. 6969. Winged nucleus. 



N. G. C. 6401. Star-clusters. N. G. C. 7722. Spiral with absorption streak. 



With the 100-inch telescope Mr. Pease inade direct photographs of 

 N. G. C. 598, 2022, 2274, 3379, 3395-6, 4656-7, 7026, 7662, the Orion 

 Nebula, and Nova Aquilse. All of these nebulae are well-known ob- 

 jects and have been previously described. 



Additional photographs made by Mr. Pease with this telescope for 

 classification include N. G. C. 5655 and a small spiral, and the region 

 surrounding N. G. C. 4009, which proves to be a group of 45 or more 



