238 ANNUAL RECORD OF SCIENCE AND INDUSTRY. 



tion of a bright disk or line of light, which are capable of to- 

 tally obscuring an object of less diameter than these fringes, 

 varies as the natural sine of the half angle of aperture for the 

 same wave of light. Now the half wave-length for an ex- 

 treme aperture of nearly 180 is, upon this theory, the y-^TuT 

 of an inch, which approximates very closely with the meas- 

 urements of that very difficult test diatom, Amphipleura 

 pellucida. It would seem, therefore, that the statements of 

 Mr. Webb as to the minuteness of the letters in the Lord's 

 Prayer, as mentioned above, must be erroneous. Those who 

 are interested in the question of the present limits of vision 

 will find many useful hints in Dr. Royston-Pigott's paper in 

 the above-named journal, and from which we extract the fol- 

 lowing notice of the application of the formula of Helmholtz. 

 From the experiments of Mr. Broun, F.R.S., who found that 

 a dark-brown hair 0.0020 inch wide and 2.5 inches long: was 

 visible by a young eye against a northwest sky at thirty-six 

 feet distance, subtending at that time 1^ seconds of arc, it 

 follows that such an eye can actually see lines on glass 

 loioo of an inch wide and -^ long ; and if such be the power 

 of the naked eye without a lens, it ought to follow that the 

 Trnnnju" f an inch ought to be seen by the same eye with 

 a power magnifying ten times. Now Nobert's lines, if the 

 interspaces are the same width as the lines, when they are 

 112,000 to the inch, would have an absolute diameter of 

 -77 g 4 1 of an inch; and as such a line would at ten inches dis- 

 tance subtend one ninth of a second nearly, a magnifying 

 power of a little over five hundred diameters should make a 

 visual angle thirty times greater than Mr. Rroun's result 

 above stated. We must conclude, therefore, with Dr. Pigott, 

 that although theoretically, and for brilliant lines and points, 

 the separable interval may be for the widest aperture half 

 a wave-length, yet when by proper precautions of illumina- 

 tion the diffraction can in a great measure be destroyed, the 

 limit is much smaller than the one assigned. 



APPARATUS AND OBJECTIVES. 

 In the American Naturalist for December, Dr. R. IT. Ward 



briefly reviews the exhibition of microscopes at the Centen- 

 nial Exhibition. The Continental microscopes were chiefiy 

 re pre sen ted by the exhibit of Nachet, the English depart- 



