16 REPORT — 1853. 



In the great mirror of Lord Rosse (6 feet in diameter), having a sidereal focus 

 of 52 feet, I saw a moon-image, of extraordinary beauty, or rather magnificence, 

 nearly 6 inches across. The light received on this image (supposing the loss by 

 reflexionequalto that by refraction)* was -^ of that on mine, so that the picture 



might probably be impressed on a collodion surface in one-fourth or even one-sixth 

 of the time required on mine ; or in the same time as on mine, it would give a twice 

 magnified image (-v/'i), viz. a moon 12 inches across. I confidently believe that 

 the master of this mighty engine will make it do its work. 



I now turn to a different view of the subject, which is, however, of fully equal 

 importance ; viz. the nature of the movement by which the telescope must be 

 made to follow the moon. The clock now usually employed, with centrifugal 

 balls, I find quite equal to follow star, sun, or moon, by an easy variation of its 

 rate. The moon's motion in her orbit is variable, but not so much variable as to 

 require in a few minutes any differential rating of the clock set by trial to her 

 mean rate for the hour. It must, however, be accurately set to this rate, for, 

 otherwise, in direct proportion to the magnifying power, will be the brush or 

 indistinctness of every meridional outline, and the equatorial extension of ever)' part 

 of the picture by an angular quantity (in) expressing the clock error. The moon 

 has never, at two succeeding moments of time, the same declination ; and except 

 about the epochs of greatest north and greatest south declination, her change of de- 

 clination is sensible in a few minutes. Except at these times the change of her 

 declination is sensible in the picture obtained by an exposura of even 5 minutes ; 

 as may be seen by the photograph of 15th July, where the north and south edges 

 are brushed, and the craters appear elongated in a meridional direction, the western 

 edge remaining quite sharp. This difficulty might be practically overcome by a 

 piece of mechanism connected with the clock, giving to the telescope a slow motion 

 in declination (-h or — ) proportioned, in a given short time, nearly to the number of 

 hours from the nearest epoch of greatest north or greatest south declination. 



The image obtained by the photographer should not only be perfect, but must be 

 taken on a surface quite fine and true, so as to bear magnifying by eye-glasses. In 

 this particular, at present, only the silver-plate and the collodion film on glass have 

 claims to approbation. 1 am not able to report at present the possession of such 

 perfect images, as to bear any but very low magnifiers ; but this imperfection of the 

 images will probably diminish or vanish by further trials, or by the aid of more 

 fortunate experimentalists. 



Supposing our photographic power to be raised so much as to copy on silver, 

 glass, or paper, all that the lens can show, what will be the picture presented under 

 a magnifying eye-glass ? Let us assume in the case of Lord Rosse's telescope, 

 a first image of 12 inches in diameter, and that it will bear magnifying eight times. 

 This will be equivalent to 96 inches diameter for the moon, and about — of an 

 inch for a mile. The physical maps of Yorkshire which I now exhibit in comparison 

 are on nearly the same scale (— th of an inch to a mile), and if inspected at a 

 distance of 10 inches will give a fair notion of the apparent magnitudes of objects on 

 the moon on this condition, which nearly expresses a magnifying power of 1000. 

 It is obvious therefore that by such means we may have a record of the moon's 

 physical aspect under every phase of illumination, under every condition of libration, 

 nearly as we should see her at a distance of 240 miles, undimmed by more than a 

 few miles of the strata of the earth's atmosphere. We should see and measure on 

 the glass or the metal, her mountains and valleys; her coasts and cliffs; her glens 

 and precipices ; her glacial moraines, escars and sand-banks ; her craters of eruption, 

 of upheaval, or explosion ; her lava streams, and the scattered heaps projected from 

 the interior. We should spy out the various actinic powers of the difl'erent parts 

 of the surface, compare these with theiV obvious reflective powers, and thus come to 



* The loss of illuminating power is greatest by reflexion; but there is no course of expe- 

 riments known to me from which it can be determined what is the proportionate loss oi pho- 

 tographic power in reflectors and refractors. It seems probable that reflectors should be more 

 efficacious than achromatics, which are suited, as mine is, to astronomical observation. 



