55Q 



NOTES AND QUERIES. 



[No. 188. 



If any one has a lens, the chemical and visual 

 focus being different, his only remedy is M. 

 Claudet's method. And this method will also 

 prove better than any other way at present known 

 of ascertaining whether a lens will take a sharp pic- 

 ture or not. If, however, any plan could be de- 

 vised for making the solar spectrum visible upon 

 a sheet of paper inside the camera, it would reduce 

 the question of taking sharp pictures at once into 

 a matter of certainty. 



All lenses, however, should be tried by the op- 

 ticians who sell them ; and if they presented a 

 specimen of their powers to a buyer, he could see 

 in a moment what their capabilities were. 



Weld Taylor. 



Bayswater. 



Photography and the Microscope ("Vol. vli., 

 p. 507.). — I beg to inform your correspondents 

 K. I. F. and J., that in Number 3. of the Quarterly 

 Journal of Microscopical Science (Highley, Fleet 

 Street) they will find three papers containing 

 more or less information on the subject of their 

 Query ; and a plate, exhibiting two positive pho- 

 tographs from collodion negatives, in the same 

 number, will give a good idea of what they may 

 expect to attain in this branch of the art. 



Practically, I know nothing of photography ; 

 but, from my acquaintance with the modern 

 achromatic microscope, I venture to say that pho- 

 tography applied to this instrument will be of no 

 farther use than as an assistant to the draughts- 

 man. A reference to the plates alluded to will 

 show how incompetent it is to produce pictures of 

 microscopic objects : any one who has seen these 

 objects under a good instrument will acknowledge 

 that these specimens give but a very faint idea of 

 what the microscope actually exhibits. 



It is unfortunately the case, that the more 

 perfect the instrument, the less adapted it is for 

 producing photographic pictures ; for, in those of 

 the latest construction, the aperture of the object- 

 glasses is carried to such an extreme, that the 

 observer is obliged to keep his hand continually 

 on the fine adjustment, in order to accommodate 

 the focus to the different planes in which different 

 parts of the object lie. This is the case even with 

 so low a power as the half-inch object-glasses, 

 those of Messrs. Powell and Lealand being of the 

 enormous aperture of 65° ; and if this is the case 

 while looking through the instrument when this 

 disadvantage is somewhat counteracted by the 

 power which the eye has, to a certain degree, of 

 adjusting itself to the object under observation, 

 how much more inconvenient will it be found in 

 endeavouring to focus the whole object at once on 

 the ground glass plate, where such an accommo- 

 dating power no longer exists. The smaller the 

 aperture of the object-glasses, in reason, the better 

 they will be adapted for photographic purposes. 



Again, another peculiarity of the object-glasses 

 of the achromatic microscope gives rise to a 

 farther difficulty; they are over-corrected for 

 colour, the spectrum is reversed, or the violet 

 rays are projected beyond the red : this is in order 

 to meet the requirements of the eye-piece. But 

 with the photographic apparatus the eye-piece is 

 not used, so that, after the object has been brought 

 visually into focus in the camera, a farther ad- 

 justment is necessary, in order to focus for the 

 actinic rays, which reside in the violet end of the 

 spectrum. This is effected by withdrawing the 

 object-glass a little from the object, in which 

 operation there is no guide but experience ; more- 

 over, the amount of withdrawal differs with each 

 object-glass. 



However, the inconvenience caused by this 

 over-chromatic correction may, I think, be reme- 

 died by the use of the achromatic condenser in 

 the place of an object-glass ; that kind of con- 

 denser, at least, which is supplied by the first mi- 

 croscopic makers. I cannot help thinking that 

 this substitution will prove of some service ; for, 

 in the first place, the power of the condenser is 

 generally equal to that of a quarter of an inch 

 object-glass, which is perhaps the most generally 

 useful of all the powers ; and again, its aperture 

 is, I think, not usually so great as that which an 

 object-glass of the same power would have ; and, 

 moreover, as to correction, though it is slightly 

 spherically under-corrected to accommodate the 

 plate-glass under the object, yet the chromatic 

 correction is perfect. The condenser is easily de- 

 tached from its " fittings," and its application to 

 the camera would be as simple as that of an or- 

 dinary object-glass. 



However, my conviction remains that. In spite 

 of all that perseverance and science can accom- 

 plish, it never will be in the power of the photo- 

 grapher to produce a picture of an object under 

 the microscope, equally distinct in all its parts ; and 

 unless his art can effect this, I need scarcely say 

 that his best productions can be but useful aux- 

 iliaries to the draughtsman. 



I see by an advertisement that the Messrs. 

 Highley supply everything that is necessary for 

 the application of photography to the microscope. 



■ Rectory, Hereford. 



In reply to your correspondent J., I would ask 

 If he has any photographic apparatus ? if so, the 

 answer to his question " What extra apparatus is 

 required to a first-rate microscope in order to 

 obtain photographic microscopic pictures ? " would 

 he None; but if not, he would require a camera, or 

 else a wooden conical body, with plate-holder, &c., 

 besides the ordinary photographic outfit. Part III. 

 of the Microscopical Journal, published by High- 

 ley & Son, Fleet Street, will give him all the in- 

 formation he requires. 



