534 



MICEOSCOPE. 



the maximum angle, as far as possible, with the 

 lower powers, such as the T \th and th inch 

 objectives. These are now frequently made 

 with angles, ranging from 110 to 140, for 

 direct light, and 140 to 160 for oblique, and 

 possessing most exquisite definition. It has 

 been found by the American artists, that low 

 power objectives of these high angles, cannot 

 be made equally perfect, at the same time, for 

 direct and very oblique light. To meet this 

 difficulty, Mr. Tolles supplies an extra front, 

 to be used with extreme oblique illumination, 

 and by thus changing fronts, the objective is 

 made perfect as possible, and with very little 

 trouble. The extra front when not in use, is 

 contained in the objective box, the bottom of 

 which unscrews to receive it. Messrs. W. 

 "Wales & Co. of Fort Lee, Bergen Co., New 

 Jersey, whose objectives are now considered as 

 of the highest quality, make the correction, as it 

 appears to us, in a more efficient manner, by 

 means of an extra bade, which is introduced 

 with but little more trouble than the extra 

 front, producing a most perfect and satisfactory 

 correction. "We have a th of their make, thus 

 corrected, now before us ; and after comparing 

 it with the best glasses of Ross, Powell, & Le- 

 land, Smith, Beck & Beck, Spencer and Tolles, 

 have decided it to be as finely corrected an ob- 

 jective as has yet been made; the angle for 

 direct illumination being about 140, and for 

 oblique near 160, performing in the most satis- 

 factory manner upon Podura, and easily resolv- 

 ing the GrammatopTiora subtillissima of Provi- 

 dence ; we are the more particular to mention 

 this as we omitted to name Messrs. "Wales & Co. 

 among the American opticians in our previous 

 article. 



"While the attempt has thus been successfully 

 made, by both the American and English artists, 

 to push the -j^th inch and th inch objectives, 

 ordinarily, up to 120 and 140, or more, there 

 has been a slight decrease in the angles given 

 to the higher powers, in order to obtain an in- 

 creased working distance as a general rule, 

 the lower powers, such as the |th inch have, 

 for a given angle, a much greater working dis- 

 tance than a higher objective with the same 

 angle. The $th inch of Wales, above alluded to, 

 has a working distance of over ? \th of an inch, 

 while an earlier -^th inch of Spencer, of fine 

 corrections, and of only 160 of angle or there- 

 abouts, cannot be used at all through glass of 

 7rt T th of an inch in thickness. There can be no 

 question that amplification produced by means 

 of an objective, is far superior to that obtained 

 by a high eye-piece, and that with equal angles 

 and magnifying power, the higher objective, if 

 well made, will surpass the lower in clearness 

 of vision. To meet the demands of physiological 

 investigators, especially, Messrs. Smith, Beck, 

 fc Beck now supply a -fa inch objective, having 

 a single lens front, and the moderate angle of 

 120. This objective is corrected, as had been 

 previously done by Nachet, upon the same 

 principle as the dialytic telescopes of Plossel and 



Frauenhofer, by means of an ovar-correction 

 to the posterior lenses. This objective is very 

 highly spoken of, and has a very good working 

 distance. Messrs. Powell & Luland, already 

 justly celebrated for their fine -^th inch have 

 produced a ^ inch, also a 7 y h inch of extreme 

 angles; the latter is said to bear a power of 

 9,000 linear on the Podura with the utmost 

 distinctness, and to have made revelations as to 

 the ultimate structure and disposition of nerve 

 fibre, unsuspected by other objectives. 



Mr. Tolles has supplied as high objectives aa 

 3 Vth inch of most exquisitely corrected, but not 

 of extremest angle, about 165. The highest 

 objective yet sent out by "Wales & Co. is a T yh 

 of admirable performance, and large working 

 distance. It has appeared to us, that the only 

 hope of real advance in these high powers, 

 would be to make the first lens single, and a 

 gem, say of garnet ; thus giving a much increased 

 working distance, with highest amplification, 

 Messrs. Wales & Co. are about putting this to a 

 practical test. Notwithstanding the determined 

 opposition of a certain class of observers to 

 objectives of high angle, it will be generally 

 conceded by those who have had experience, 

 that there is much greater precision and sharp- 

 ness of definition, with a finely-corrected high 

 angle objective, than with the same power of 

 lower angle. We noticed in our former article 

 some of the improvements in eye-pieces, and 

 will only here allude to Mr. Tolles' new binocu- 

 lar. It is many years since binocular vision 

 was suggested and employed with the micro- 

 scope. None of these instruments appear to 

 have given much satisfaction, until Mr. Wenham 

 introduced his ingenious arrangement, which 

 is now generally employed. The arrangement 

 of prisms devised by Dr. Eiddell gave pseudo- 

 scopic vision. The writer well remembers look- 

 ing through this first instrument, which was 

 constructed by Messrs. Graunow. A better ar- 

 rangement of prisms was devised by M. Nachet, 

 whose binocular is highly commended by Dr. 

 Carpenter. Mr. Wenham's single prism was a 

 still greater improvement, the result of a per- 

 severing determination of this gentleman, to 

 whom microscopy owes so much, to improve 

 the binocular microscope to the utmost possi- 

 ble. We do not, by any means, intend to dis- 

 parage it in preferring Mr. Tolles' arrangement, 

 which, we feel quite sure, with perhaps some 

 improvements, Mr. Wenham himself will adopt, 

 after careful consideration of its advantages. 

 Not only is it applied in the same manner as 

 an ordinary eye-piece, and can thus, with the 

 smallest amount of trouble, be applied to any 

 microscope, but it it also an erecting eye-piece 

 of first quality and hence particularly useful 

 in dissections; and the division of the pencil 

 being made quite near to the eye, the injurious 

 effect of any slight departure of the faces of the 

 prisms from a perfect plane, becomes a mini- 

 mum. It is impossible that any binocular ar- 

 rangement shall quite equal in definition the 

 monocular form, however perfect may be the 



