August 18, 189?. 



SCIENCE. 



95 



the difference in the objects, a strong mental effort is re- 

 quired to disregard the impression in one eye, and fix the 

 attention upon the other only. Again, when we close one 

 eye by the contraction of the orbicular muscle, or by 

 pressure, as Avith the hand, we cause contraction of the 

 accommodating muscle, also, and that of the open, occu- 

 pied eye, as well. I have proof of this many times each 

 day while examining eyes by the ophthalmoscope; but 

 we are all familiar with ths spasm in both eyes when a 

 particle of dust is beneath the lid of one only; and, again, 

 we are conscious of an effort amounting almost to an im- 

 possibility, before training, of keeping one eye open and 

 the other shut. 



Both these conditions are present and are factors in the 

 fatigue which accomj^anies the use of a monocular instru- 

 ment, and are strong reasons for employing a binocular 

 one, when possible. Of course, each form has its own 

 especial use and place, but this is not our present pur- 

 pose to discuss. It is to overcome these sources of 

 fatigue in the use of the monocular instrument that an 

 eye-protector is used. 



"\^'hen anything is placed far within the focus of an eye 

 no image of it is formed upon the retina, and it becomes 

 invisible. If, then, it should be opaque and large enough 

 to cover the whole field of vision, it is not only invisible, 

 but shuts off the sight of all other objects as well, leaving 

 the mind free to attend to the image on the retina of its 

 fellow. On this principle quite a number of devices have 

 been proposed and used, among which a plain card, per- 

 forated and slipjDed upon the tube, has been, perhaps, the 

 most frequent. This has to be placed low down in order 

 to be out of the way of the face, and thus requires to be 

 so large to cover the field of vision that it hides the 

 stage and interferes with the adjusting screws. 



^'^ 



Another consists of a small plate extending horizontally 

 from the cap of the ocular. In this the edge must be cut away 

 to admit the bridge of the nose. This gives it a curved 

 form, and prevents its being used before each eye alter- 

 nately, except by removal and inversion. It must also be 

 removed with each change of ocular. These removals and 

 replacings demand so much time that most workers think 

 it hardly worth the trouble. 



The form that I have found satisfactory, after use for 

 several years, consists of a small disc of blackened brass, 

 about the size and shape of a spectacles glass, and sup- 

 ported near the eye by a wire extending from its outer 

 margin obliquely downward to a j)oint on the tube low 

 enough to be out of the way of the nose, then bent up- 

 wards, parallel to the tulie, but not touching it, and at- 

 tached to a cut-ring which clasps the top of the draw tube 



beneath the ocular. The accomjjanying drawing shows' it 

 ia place, and will need no further explanation. 



The advantages of this form are: First — The small size 

 of the disc and sujsport interfere the least possible with 

 the adjusting screws and view of the stage. Second — It 

 is easily adjusted to the eye-distance of any worker. 

 Third — It is not in the way of the nose. Fourth — It can 

 be easily swung around before either eye, without re- 

 moval. Fifth — It is not disturbed in changing oculars. 

 Sixth — Any mechanic can make one at a small expense. 

 The one I am using was made by Zeutmayer, of this city. 



LETTERS TO THE EDITOR. 



^"^Correspondents are requested to be as brief as possible. The 

 writer's name is in all cases required as a proof of good faith. 



On request in advance, one hundred copies of the number con- 

 taining his communication will be furnished free to any corres- 

 pondent. 



The editor will be glad to publish any queries consonant with 

 the character of the journal. 



Birds That Sing by Moonlight. 



T.1E reading of the very interesting article in Science 

 for Dec. 2, entitled "Birds That Sing in the Night," by 

 Morris Gibbs, brought vividly to my mind the pleasure I 

 have felt in listening to nocturnal bird music. 



The bii-ds which I have most frequently heard sing at 

 night here in southeast Kentucky are of different species 

 from those mentioned by Mi-. Gibbs, prominent among 

 them being the Oven-Bird {Seiurus aurocapillus), which, 

 although I have never seen any mention of the fact in 

 print, sings regularly on moonlight nights. 



On svich occasions the song is usually the extatic, quiv- 

 ering jumble of warble and twitter so often heard from 

 this bird at dusk, when he flies in zig-zag lines and short 

 curves up above the tops of his native woods, and as 

 quickly descends, all the time bubbling over with melody. 



Almost every bright moonlight night in spring and 

 early summer this song may be heard at intervals, break- 

 ing with silvery sweetness into midnight's tranquility. 



Another bird, often heard on moonlight nights, 

 though by some it is not considered worthy the name of 

 a song, is that of the Yellow Breasted Chat {Icteria virens). 



The Cuckoos are also often heard by moonHght during 

 theii- southward migrations after all the resident individ- 

 uals have departed. 



I have frequently noticed that a bright fire in or near 

 the woods at night called forth sleepy chirps and snatches 

 of song from various species of birds. John B. Lewis. 



The Cambojan Khmers. 



Having some time ago carefully studied the question of 

 the origin of the Khmers of Cambodia, and the result of 

 my enquiries having been jiublished in the Bevue d'An- 

 thropologie (3rd Ser. Vol. I, 1886, 2d fasc), under the 

 title of Les Camhodgiens et lew origine, I may j)erhaps be 

 allowed to make some remarks on Prof. A. H. Keane's 

 letter which a25peared in Science for August 4. That the 

 Khmers belong to the white race, whether this be called 

 Caucasian or not, cannot well be denied, and Mr. Keane 

 is doubtless entitled to the credit of having first pointed 

 out the fact. But that the Khmers are, as he states, true 

 aborigines in the country where they are now found is 

 very questionable, and indeed the best I'rench authorities 

 agree with Dr. Maurel in deriving them from India. The 

 date of their- arrival in Cambodia is given by M. Moura, 

 and is fixed by the annals of the ancient Cambodian em- 

 pu-e as having taken place about 513 B. C. According to 

 the view elaborated in the paper above referred to, the 

 ancestors of the Khmers were allied to the Tandavas of 

 the Hindu epic, the Mahabharata, and I have endeavored 



