March 9, 1883.] 



♦ KNOWLEDGE • 



149 



relation of things by a mere outline sketch, which, of 

 course, cannot be considered in any sense a repregenlation, 

 since it fails entirely to give an idea of the shading and 

 gradation of light The head of the comet presented no 

 detinite outline whatever, and the nucleus very little. The 

 knots were mere condensations of brightness in the midst 

 of diffuse light. When the dawn came on, the fainter 

 parts successively disappeared, so that at a certain stage 

 tlie nucleus seemed to be divided into two portions. A 

 small telescope would probably show things in the same 

 way even before dawn, and this is undoubtedly the origin 

 of the reports that the comet had split in two. 



This great and unprecedented elongation of the nucleus 

 is a most remarkable phenomenon. If it had occurred at 

 or near the time of perihelion passage, it might have been 

 naturally attributed to the divellent action of the sun's 

 attraction ; but it is a little ditlicult to see why the thing 

 should have pulled out and come to pieces in such a way 

 after getting safely by the crisis. It is worth noting that 

 this peculiarity of the comet adds greatly to the difficulty 

 of making accurate observations of its position : one does 

 not know just upon what point to direct his instrument 



Continuous cloudy weather prevented any observation of 

 the comet until the 1 Oth. On that date the appearance of 

 things as seen in the great equatorial was very much what 

 it had been on the 10th with the smaller telescope. There 

 were no envelopes, and the only "jet" was the bright 

 streak following the nucleus. The dark stripe had wholly 

 disappeared, as if obliterated and replaced by the bright 

 one. The " knots " in the nucleus were seen to be irregular 

 in form, and were arranged not in a straight lino but in a 

 somewhat broken curve, conforming to the curvature of the 

 tail, which at this time extended 18°, and was fully 

 60,000,000 miles in length. The bright stream originated 

 not at the extremity of the nucleus, but came out tangen- 

 tially from the convex side, and perhaps had its source in 

 the largest of the knots, which was now the third from the 

 sunward extremity. The whole length of the nucleus 

 measured 48^", corresponding to a length of more than 

 40,000 miles, the diameter of the largest single mass being 

 about 5,000 or 6,000 miles. The only other observation 

 •we have been able to make at Princeton was nine days 

 later, on October 24th. No material changes were noticed, 

 though the comet was very much fainter. The same 

 lengthened granular nucleus continued, and seems likely to 

 persist until the comet disappears. 



(To he continued.) 



HOW TO USE OUR EYES.— 11. 

 Bv John Browning, F.R.A.S. 



{Coniinueil ''om page 112.) 



THERE is good reason for be.K\ing that the eye takes 

 a photograph of every object looked at intently. 



Rabbits have been held before a window for a few 

 seconds and then killed. A picture of the window has 

 been found on their retina. 



The impression on the retina is generally said to last 

 about the sixteenth part of a second, but this depends upon 

 the brilliancy of the object and the length of time we look 

 at it. 



Xow for some important hints for preserving your eye- 

 sight Never look at an intensely bright light for any 

 length of time, or a permanent image may be formed on 

 the retina. Avoid, most carefully, exposing the eyes to a 

 very bright light after they have been' in darkness, as such 

 changes are injurious, and have been known to produce 

 blindness. 



When using a microscope, always incline it as much as 

 possible towards the horizontal direction. Most persons 

 use the microscope almost upright. Looking down into 

 the microscope in this position gorges the eye with blood 

 by stopping the circulation in the nock. 



A Newtonian reHecting telescope is very superior to an 

 achromatic telescope for those who have any fear of 

 straining their eyes, as the stars and other heavenly 

 bodies can be seen best with this kind of telescope when 

 they are directly overhead. 



If a person wishes to look at them when they are thus 

 situated, he must lie downi on his back if he uses an 

 achromatic telescope. With a reflecting telescope the 

 celestial objects may be viewed when they are overhead by 

 the observer looking horizontally into the telescope. 



When adjusting a very bright lamp flame, if you wish to 

 do it slowly, look at the flame through a slit formed by 

 almost closing two of the fingers. This will protect the 

 eyes greatly. Always turn your back to the source of light 

 when you are reading, so that the light may fall on to the 

 book, instead of coming into your eyes. 



'/ 



■^i^ 



t .R. 4. 



Always lean well back when reading, and hold the'boofr 

 up, as shown in the engraving (Fig. 3). Do not lean 

 forward and face the light, as in thi'? view (Fig. 4). 



