232 



SCIENCE. 



[V^OL. VIII., No. 188 



by 1,000,000, at the distance of 100 feet it will be but 

 1. It is thus seen that the effects are intensely local, 

 and but comparatively trifling at even short distance. " 



The wide-spread damage, in the Chicago explosion 

 was undoubtedly due, in a much larger degree, to 

 the gunpowder than to the dynamite exploded. 



Another fact and deduction relating to the escape 

 of several magazines near the great explosion are 

 quite as misleading, if not as erroneous, as the former 

 ones. 



If we are correctly informed, most, if not all, of 

 the magazines nearest the exploded buildings, con- 

 tained dynamite. Now, it is a fact well known to 

 experts that this material is non-explosive by shock 

 or by fire applied separately, but requires some ful- 

 minate combining both concussion and combustion, 

 acting simultaneously, to explode it. Hence, being 

 protected from the fire or combustion of the explo- 

 sion by the walls of the magazines, and being unsus- 

 ceptible to the force of the concussion, there is nothing 

 remarkable in the salvation of the adjacent maga- 

 zines. Even those, if any, which contained gun- 

 powder (that explosive being protected from contact 

 with fire, and remaining inactive) were uninjured 

 for equally scientific reasons. 



The article concludes, referring to its statements 

 and deductions, by saying, "These are facts which 

 could not have been exemplified save at much cost 

 and risk, and our government officers and other men 

 of science will, we may be sure, bear them carefully 

 in mind hereafter." 



Now, as we have shown that the above statements 

 are not facts, but that the contrary is the real truth, 

 and as the actual facts have been ascertained as well 

 by many of our government officers as by a large 

 number of experts all over the world, we would re- 

 spectfully suggest to the Herald, when it intends to 

 publish another scientific dynamite article, that it 

 secure the services of, or at least submit its facts to, 

 some such expert as General Abbott or Gen. John 

 Newton, both of the U. S. army, whose experience 

 with explosives of every kind has been exhaustive, 

 and thus obtain information that the public can rely 

 o°- A. W. G. 



New York, Sept. ]. 



On a means of determining the limits of distinct 

 vision. 



If an image (A'B') of an object {A B) be thrown on 

 a screen by means of a lens (L, for simplicity sup- 

 posed free from spherical aberration), and the screen 

 moved forward or backward, the image will be 

 blurred. If part of the rays be stopped by a dia- 

 phragm (D L)'), this blurring will be less as the aper- 

 ture of D D' diminishes, for this lessens the spaces 

 {W Z, etc) over which the rays from any one point 

 of the object are spread on the screen. Now, let the 

 rays be cut off from one side alone ; let a curtain (D) 

 descend from above. The upper boundaries {W, X, 

 etc.) of the spaces WZ, XY, etc., will descend, while 

 the lower ones remain stationary. If the object be 

 dark against a brilliant background, the light from 

 above A will be cut off as B descends, and the blurred 

 edge {XY) of the image becomes dark ; so that, in the 

 limit, instead of a blurred image {WY), there would 

 be a distinct one {Z Y), or, as the image of D ascends, 

 the image oi A B appears to move to meet it, the part 

 near D leading the way, since D intercepts the ex- 

 treme ray from A before that from B. 



If the object be light on a dark ground , the effect 

 will be most apparent on the boundary farthest from 

 D, since the blurred edge that changes to dark is 

 more noticeable than that which changes to light. 

 If the image be formed in front of the screen, mak- 

 ing the blurred image Z X , a little consideration 

 will show that the apparent motion of the image will 

 always be away from the image of D. 



These results may be verified with any lens, but 

 are most strikingly shown with the eye, using a sheet 



of paper close to the eye as curtain, and any object, — 

 as a pin, pencil, or ruler, — seen against a window or 

 lamp as background. A slit in a piece of paper held 

 against a lamp serves as light object on a dark ground. 

 It is, of course, easy to hold the object so near that it 

 will be blurred ; but special effort may be required 

 to blur a distant object, except with near-sighted per- 

 sons. The applicability of this in making a test of 

 the limits of distinct vision is now apparent. Let a 

 ruler lean against the shade of a lamp ; place the 

 eye so near that the image is necessarily blurred, and, 

 moving the edge of a sheet of paper back and forth 

 before the eye, step slowly backward till apparent 

 motion of the object ceases ; continue the backward 

 movement until the object begins to recede slightly 

 from the screen : the space where there was no 

 motion is that in which alone distinct vision is pos- 

 sible. Of course, every effort must be made to 

 accommodate the focus of the eye to the object dur- 

 ing the whole experiment. 



It is a more difficult task than one thinks, to decide 

 by simple judgment whether an object is seen dis- 

 tinctly or not, except it be much blurred. If the 

 image is fairly distinct, most people will suppose it to 

 be perfectly so. The test described above never fails 

 to show whether or not the judgment is correct. 



The effect noticed above also adds to the appear- 

 ance seen when two networks of thread or wire not 

 in the same plane are held before the eye. The 

 watered appearance is of course due to curves which 

 are the loci of the intersections of one set of wires 

 with the other ; but these intersections are made 

 noticeable by the fact, that, when two wires not in 

 the same plane and making an acute angle are held 

 before the eye, the nearer acts the part of the cur- 

 tain D in the above demonstration, and an irregular 

 dark spot is seen about the point where the wires 

 cross. The writer hopes to make a series of experi- 

 ments as to the limits of distinct vision in different 

 persons, using the test suggested above. Its sim- 

 plicity, and the absence of any judgment on the part 

 of the person experimented upon, other than as to 

 the direction of motion of the object, commends it to 

 the investigator. Arthur E. Bostwick. 



Montelair, N. J., Aug. 30. 



