BIBLIOGRAPHY OF IIERSCHEL's WRITINGS. 585 



Herschel, W.: Synopsis of the Writings of— Conlimu'd. 



A. D. Vol. P. 



1805 95 33 These objects were obscivcd at a distauce of 2407.85 inches from tho 

 mirror of a ten-feet reflecting telescope. It was found that in the 

 telescope a body subtending the niagnitied angle of 2' 19" could 

 easily bo recognized as round. 

 2d Experiment, ivitli small Globules of Sealing-wax. 



34 It appeared that with a substance not reflecting much light the mag- 



nified angle must be between 4 and 5 minutes before we cau see it 

 round. 



35 3d Experiment, ivith Globules of Silver. 



36-57 4th Experiment, with Globules of Fitch, Bees-wax, and Brimstone. 

 38-39 5th Experiment, xvith Objects at a greater Distance. 

 40 6th Experiment, with illuminated Globules. 



40 Spurious Diameters of Celestial Objects. 

 Observations and Experiments, %vith Bemarks. 



41 With stars, the spurious diameters are larger than the real ones, which 



are too small to be seen. 

 The spurious diameters of stars are of difierent sizes, and under the 



same circumstances their dimensions are of a permanent nature. 

 The spurious diameters of the stars are differently coloured. 



42 Their spurious diameters are lessened by increasing the magnifying 



power, but in a much less ratio. 

 Magnifying power acts less on the large diameters and more on the 

 small ones. When the aperture of the telescope is lessened, it will 

 occasion an increase of the spurious diameters ; but this increase is 

 not proportional to the diameters of the stars, acting more upon tho 



43 small spurious diameters and less upon the large ones. Very small 

 stars, however, lose their spurious diameters, and become nebu- 

 lous. 



The spurious diameter of a star is reduced by haziness of the atmos- 

 pbere. At a low altitude a star makes a spectrum, being coloured 

 by the prismatic power of the atmosphere. 



44 Spurious diameters of terrestrial objects, with similar re- 



marks. 

 7th Experiment with Silver Globules. 



45 8th Experiment. 



The luminous spots or spurious disks of the gloT)ules were of uneiiual 



diametei's. 

 9th Experiment. 10th Experiment. 



46 11th Experiment. 12th Experiment. 



47 13th Experiment. 14th Experiment. 

 48-49 15th Experiment, with Drops of Quicksilver. 



50 [These experiments, from tho 7th to the 15th, inclusive, prove that all 



the remarks made concerning the spurious disks of stars are also 

 applicable to sufficiently small and bright terrestrial objects.] 

 16th Experiment, with black and white Circles. 



51 17th Experiment, with different Illumination. 

 18th Experiment. Measures of spurious Disks. 



With drops of quicksilver, the spurious disks were measured by means 

 of distant disks of known diameters upon which they were projected. 

 By covering the mirror with screens of different apertures, and also 



52 by stopping out its center by circular screens of different size, it 

 was found that the sizes of the spurious disks were not determined 

 by the quantity of light reflected from the mirror, but rather by the 



