TRANSACTIONS OP THE SECTIONS. 15 



known or supposed to be variable, — the law or rate of such variation being put as a 

 problem to be determined. The moon, our friendly satellite, is exactly in the condi- 

 tion to require this kind of investigation ; and if photography can ever succeed 

 in portraying as much of the moon as the eye can see and discriminate, we 

 shall be able to leave to future times monuments by which the secular changes 

 of the moon's physical aspect may be determined. And if this be impracticable, if 

 the utmost success of the photographer should only produce a picture of the larger 

 features of the moon, this will be a gift of the highest value, since it will be a basis, 

 an accurate and practical foundation for the minuter details, which, with such aid, 

 the artist may confidently sketch. 



When, therefore, at the Ipswich Meeting of the Association, the 2-3-inch Daguer- 

 reotype of the full moon, which had been taken by Professor Bond from the great 

 Achromatic of Cambridge, U.S., was shown to astronomers, their gratification was 

 extreme. Humboldt possesses one of these curious light-pictures of the moon, of 

 2 inches diameter, prepared by Mr. Whipple, of Boston, U.S., in which the so-called 

 seas and annular mountains are clearly distinguished*. 



The Committee, to whom the Association, at its Belfast Meeting, committed a 

 Survey of the Physical Aspect of the Moon, were not negligent of this powerful aid 

 to an accurate drawing. The great telescopes of Birr, which in regard to light, defi- 

 nition, and steadiness, offered the greatest temptations to this trial, were at the 

 disposal of the Committee ; and to them, and the genius of their noble owner, we 

 must probably look for photographs of the moon on the largest scale, and with the 

 deepest contrast of light and shade. But they are not yet mounted equatorially, and 

 in the mean time I thought it useful to try the power of my own 6^-inch achromatic, 

 the work of our excellent artist Cooke, which is driven equatorially by very equable 

 clock movement in the open air. 



Before my attempt was made, some trials were made by Mr. De la Rue and others, 

 but I am not able to say what is the value of their results. 



Though prepared in some degree for this experiment in the commencement of this 

 year (1853), it was not-^ill the middle of July that I was able to submit an excited 

 collodion surface to the concentrated rays of the moon. On the 15th and 18th of 

 July, with my friend Mr. Bates, 1 obtained the pictures now presented for considera- 

 tion. They prove beyond a doubt that the research is of a useful and practicable 

 kind, and, if I mistake not, will be followed by far better things. 



In the expectation that this will become a favourite object of inquiry among phO' 

 tographers, I solicit a few minutes' attention to some of the conditions of the problem, 

 for, without a right notion of the thing to be done, much disappointment will attend 

 the trials. 



First, it must be remembered that, as moon-light is fully 100,000 times weaker 

 than sun-light, and only appears to us bright in consequence of the general dark- 

 ness around, photographs can only be taken quickly by very sensitive surfaces. The 

 moon's image in the telescope has not, indeed, really more actinic effect on the silver 

 surface than some of the duller terrestrial objects which are slowly depicted in the 

 camera. On a highly sensitive collodion, the feeblest radiants operating for the 

 shortest time produce some effect ; but firm impressions can only be had by the 

 integration of these differential quantities. In the telescope which I employ, with a 

 sidereal focus of 1 1 feet, the moon's diameter, as traced on the collodion, is about 

 li incht ; and the aperture being 6^ inches, the light of the moon's image is aug- 

 mented about 26 times as compared with the brightness of the object seen directly 

 by the eye. The time required for this image to be firmly impressed doei not 

 exceed 5 minutes, when the moon has a maximum south declination, and an 

 elevation of only 12°. 1 think it probable that when her declination is at a maxi- 

 mum to the north, and I employ the most sensitive collodion, she will draw her 

 own likeness in my camera in 1 minute, with sufficient firmness for printing. | 



* Kosmos, iii. part 2. 362. 



t The moon's mean diameter being -j-j-pth of her mean distance from the earth, the 

 mean diameter of her image on my collodion plate would be r2 inch, but the actinic focus is 

 on the outside of the focus for white light 075 inch. 



§ Since this was written many trials have been made ; the result being that a picture, 2 

 inches in diameter, may be taken by using the Huyghenian eye-piece in 30 seconds. ] 



