Nov. 27. 1852.] 



NOTES AND QUEEIES. 



515 



posure to light in the camera. No ordinary 

 collodion will, at the present period of the year, 

 and in the unfavourable state of light which has 

 long prevailed, produce a good picture in ten 

 seconds. On the 18th of the present month, ever 

 memorable as the day of the funeral of the Duke 

 of Wellington. I found the action to be at least ten 

 times slower than on ordinary days in the spring 

 and summer. I mention this day as being one which 

 would be well and justly remembered, and as being 

 one of the most favourable for pliotographic opera- 

 tions which have lately occurred. He is also in- 

 formed that the development should be carried on 

 until all the details of the picture, the pupils of the 

 eyes, the hair, &c., are well defined : this is always 

 ascertained by holding it over a piece of paper, or 

 other white surface. If the picture has not been 

 sufficiently exposed in the camera, then a decom- 

 position will often take place during the lengthened 

 time required for development, which entirely 

 spoils the effect. The protonitrate of iron, how- 

 ever, always produces a much more agreeable 

 picture than the protosulphate ; and mixed with 

 the definite quantity of pyrogallic acid solution, as 

 communicated in a late No. of " N. & Q." by my 

 friend Mr. Kater, I feel confident that no one 

 can fail in obtaining cheerful pictures of a pleasing 

 tint by using this means of development. Your 

 correspondent is also informed that it is quite im- 

 possible to define the strength of the old hyposul- 

 phite solution employed to produce dark- coloured 

 pictures : it is one of the results which can only be 

 obtained by experience. On fixing positives or 

 negatives I always use the same solution. Pour it 

 into a large bottle, by which the sediment is 

 allowed to subside. If the action of the fluid be- 

 comes weak, which is always ascertained by its not 

 abstracting the yellow colour of iodized paper, then 

 add to the same liquid some fresh hyposulphite of 

 soda. I may perhaps add the caution, that the 

 pan in which the pictures are immersed should 

 always be clean, as staining is very apt to occur 

 in places where the paper rests upon the earthen- 

 ware. H. W. D. 



The chemical and visual Foci of achromatic 

 Lenses. — Some time ago I obtained one of 

 *' 's" double combination of lenses, desig- 

 nated achromatic, under the impression that in 

 such combinations the chemical focus was made to 

 coincide with the visual. I find, however, that 

 such is not the case, the lens I have requiring fully 

 as much adjustment, before a defined picture can 

 be obtained, as did a common meniscus, which I 

 used formerly. I confess I cannot understand 

 why the term achromatic is employed under such 

 circumstances, unless it be to mislead purchasers. 



I should be glad to know if the above defect is 

 really a common one, exemption from which is the 

 result of accident rather than of science ? or 



whether it may not be fairly charged to unskilful 

 manufacture, — giving the purchaser the right to 

 return a lens labounng under it to the vendor, 

 as an article that fails to realise the pretensions 

 under which it was sold ? 



I am told that Voightlander's lenses have the 

 chemical and visual foci accurately in the same 

 plane ; and, if so, it seems to me far preferable to 

 incur their higher cost, than to be troubled with 

 the endless and variable adjustments of so-called 

 cheaper lenses such as mine. 



Perhaps some of your correspondents can men- 

 tion English makers whose prices are moderate, 

 and whose lenses they have found to be good. 



E. S. 



[There is no doubt but that the chemical and visual 

 foci of a properly constituted " achromatic " lens are the 

 same. In making this assertion we are well aware 

 how many eminent men of science contend for the 

 contrarj', and that there is a point out of the exact 

 focus of vision, as there is below the prismatic spectrum, 

 where the actinic rays of light are more energetic than 

 at the spot where the image is most beautifully de- 

 lineated. In order to obtain the exact visual focus, we 

 think more care is requisite than is often used, and it 

 is astonishing how differently various operators will see 

 the same subject. In a former number of " N. & Q." 

 the lenses of Messrs. Voightlander were spoken of from 

 our actual experience, and every day tends to confirm, 

 us in the justness of the remarks. It is a difficulty a 

 public journalist has to contend with to honestly re- 

 commend to his readers that which is best adapted 

 for their wants, without appearing to pufF in an in- 

 terested manner some especial maker. That there are 

 many English makers who can produce a good lens, 

 there is no doubt ; but we have not met with lens esso 

 satisfactory for our own use as those of Messrs. 

 Voightlander. In proof, it may be observed that 

 three of different sizes now before us, which we have 

 long had in use, do not in the slightest degree differ in 

 their chemical and visual foci. It has been observed 

 by some well qualified to judge, that this is not the 

 case with the larger lenses made by the same maker. 

 If so, it would merely argue that so great a degree of 

 perfection has not been attained with them, as has been 

 with the sizes in general use.] 



SRcpIte^ ta :^in0r €iutviti. 



Bishop Juxons Account of vendible Books in 

 England. — In perusing Mr. Crossley's interesting 

 Reply under the head " Works of the Learned " 

 (Vol. vi., p. 435.), it struck me that (seeing his 

 notice extends beyond periodicals strictly) he had 

 omitted a book of some importance ; I mean 

 Bishop Juxon's account of some of the most 

 vendible books in England, at the time of that 

 publication. I have got the book, but it is in 

 Scotland ; and I cannot, therefore, give you its 

 correct title or date. Bishop Juxon's name is not 

 given as the author on the title-page ; but, ac- 



