52 



THE POPULAR EDUCATOR. 



3. Podrir, to rot, is seldom used except in the second person 

 plural of the imperative, podrid; and the third person singular 

 of the imperfect subjunctive, podriria. 



When podrir is figuratively used in any other moods or 

 tenses, it is to be conjugated irregularly in the same tenses 

 and persons as servir, by changing the o of the verb-root into 

 u ; as, pudriendo, rotting. 



IMPERSONAL VERBS. 



Impersonal verbs (or unipersonal vevbs) are those which are 

 employed only in the third person singular, and having no 

 subject, take it or there with them in English ; as llueve, 

 it rains ; tronard, it will thunder ; nicve, let it snow ; hay, 

 there is, or there are ; habrd, there will be. 



1. Llover, to rain, is thus conjugated impersonally : 

 INF. Past Participle. Llovido. Gerund. Lloviendo. 



IND. Present. Llueve, it rains. Imperfect. Llovid, it teas raining. 

 Perfect Definite. Llovio, it rained. First Future. Llovera, it mil rain. 



2. Llueva, let it rain, is thus conjugated : 



SUB. Present. Llueva, it may rain. Imperfect. Lloviera, it would, 

 rain ; lloveria, it should rain ; lloviese, it might rain. First Future. Si 

 lloviere, if it should rain. 



All the impersonal verbs are conjugated like some of the 

 verbs whose conjugation has been already given ; thus, Hover, 

 it will be seen, is irregular, and is conjugated like mover in the 

 third person singular of each tense. 



Haber and hacer are often used as impersonal verbs, and are, 

 in such cases, to be rendered in English by the tenses of the 

 verb to be ; as, hay, there is, or there are ; 7io.ce, it is. 



3. Halter, to be, used impersonally, is thus conjugated : 

 INF. Post Partici2>le. Ilabido. Gerund. Habiendo, there being. 



IND. Present. Hay, or ha, there is, or (here are. Imperfect. Habfa, there 

 was, or there trere. Perfect Definite. Hubo, there icas, or there were. First 

 Future. Habra, there will be. 



4. Haya, let there be, is thus conjugated : 



SUB. Present. Haya, there may be. Imperfect. Hubiera, there u'ould be ; 

 habria, there should be; hubiese, there might be. First Future. Si hubiere, 

 i/ there should be. 



Hay, habia, and hubo are rendered in English sometimes in 

 the singular and sometimes in the plural, according as a singular 

 or plural noun follows ; thus, hay una muger que tiene calentura, 

 there is a woman who has a fever ; hay mugeres que no la tienen, 

 there are women who have it not. 



Hacer, when employed impersonally, is to be rendered in 

 English by the verb to be ; as, hace, it is ; hacia, it was ; hizo, 

 it wac, ; hard, it will be ; haga, it may be, etc. ; thus, hace frio, 

 it is cold ; hace mucho aire, there is much wind ; hace hm a, 

 there is a moon ; hace buen tiempo, it is good weather ; hace 

 diez meses que ella murio, it is ten months since she died. 



5. Placer, to please, is used impersonally in the following 

 tenses only : 



IND. Present. Place, it pleases. Imperfect. Flaeia, it uios pleasing. 

 Perfect Indefinite. Plugo, it pleased. 



SUB. Preset^. Plegue, it may please. Imperfect. Plugiera, it would 

 please; plugiese, it might please. First Future. Si plugiere, if it shonld 

 please. 



The persons of placer in the subjunctive are used only in 

 these expressions : plegue, plugiere, or plugiese d Dios ; may it, 

 should it, or might it please God ; si me plugiere, if it should 

 please me. 



There are some verbs that can be used in all the persons of 

 the tenses, and also, at times, impersonally ; as, es muy tarde, 

 it is very late ; es preciso, it is necessary ; cs menester, there is 

 necessity ; parece, it seems ; conviene, it suits ; basta, it is 

 sufficient. 



RECREATIVE SCIENCE. XVI. 



THE OPHTHALMOSCOPE, AND CERTAIN MORBID AND HEALTHY 

 PHENOMENA OF VISION. 



Mr. CARTER thus describes Laurence's Eeflecting Ophthalmo- 

 scope : " This instrument consists essentially of a piece of plate 

 glass, interposed between the eye of the patient and the source 

 of light. The arrangement will be readily understood by refer- 

 ence to Fig. 1, in which, for our present purpose, B will represent 

 the eye to be examined, F' the flame, and F the eye of the ob- 

 server ; and the eyes A, A', and the lens c, may be left wholly 



out of account. The rays of light from F' penetrate the plate 

 glass, s, and enter the eye, B, from which they return, either 

 divergent, convergent, or parallel, according to the state of 

 B'S refraction. Of these returning rays, the greater part again 

 pass through s, and regain their source ; but some portion 

 will be reflected by s to F, and will then become visible to an 

 observer. It is quite conceivable that an observer might, under 

 these circumstances, see an erect virtual image of the depths 

 of the eyeball, for which purpose he would require the aid of a 

 concave lens, if the eye under examination were myopic, for 

 some nearer point than infinite distance. As the apparatus 

 has been hitherto arranged, however, no details of a virtual 

 image have been rendered visible, and nothing has been seen 

 in this way but the red reflex of the interior of the eye- 

 ball. It is probable that the loss of light by the transparency 

 of the reflecting surface has been one cause of this imper- 

 fect success. By interposing a convex lens between the eye, 

 B, and the plate, s, a much better result may be obtained. 

 The first effect of such a lens is to concentrate the light it 

 receives from the flame, and thus increase the illumination 

 of B'S retina. Its next effect is to render the returning rays 

 convergent, and to bring them to union in or near the plane 

 of its principal focus, where they form an inverted image of 

 the parts of B'S retina from which they proceed. Diverging 

 from this image, the rays impinge upon the plate, s, and 

 are reflected by it in sufficient quantity to afford to the spec- 

 tator at F a very fair view of the optic nerve and vessels. 

 These are seen partially inverted that is to say, upside 

 down, but not displaced laterally. It is, of course, essential 

 that the convex lens and the plate-glass should be sepa- 

 rated by a greater interval than the principal focal length 

 of the former ; and the more this interval is increased, the 

 larger will be the size of the image and the fainter its illumina- 

 tion. The surfaces of the glass plate must be perfectly parallel 

 and perfectly smooth, as otherwise a separate image will bo 

 reflected from each of them, and the two images will confuse 

 one another. There will also be an image reflected backwards 

 from the lens, and visible to a spectator looking over the shoul- 

 der of the patient. By modifying the inclination of the plate, 

 s, the position of the point F may be varied, and the reflection 

 may even be thrown into the other eye of the person under 

 examination, so as to convert the apparatus into an autophthal- 

 moscope. By such an arrangement I find it easy to see with 

 either of my eyes the luminosity of the other, but I have not 

 succeeded in observing details. The acuteness of the angles of 

 incidence and reflection is a formidable difficulty in the way of 

 self-examination by such a method. The experimental instru- 

 ment employed by Mr. Laurence consists of an horizontal stem 

 about a foot long, supported by an upright stand. Two small 

 uprights slide in a groove on the upper surface of the stem, 

 and carry, one a convex lens of 2 inches focal distance, the other 

 a small square of plate glass, both so arranged as to turn upon 

 their vertical axes. The light is furnished by an Argand 

 burner, surrounded by an opaque chimney with a single small 

 opening, and the place of examination is otherwise completely 

 darkened. The patient is placed about 3 feet from the lamp, the 

 convex lens about 2 inches from the cornea, and the glass plate 

 about 4 inches from the lens. The eye, the lens, the plate-glass, 

 and the flame, must all, of course, be perfectly level, and their 

 centres must be the same straight line. I have described the 

 apparatus at some length, because it appears likely, if it can 

 be brought nearer to perfection, to render important aid in the 

 instruction of students. The reflection is visible to more than 

 one observer at once, and may, perhaps, hereafter be rendered 

 visible to a class. At present the matter is entirely in its 

 infancy, and nothing but the principle can be considered as 

 established. This idea was suggested to Mr. Laurence by the 

 celebrated ' Ghost, which is produced in a very similar manner." 

 It is in the diseased state of the eye that some very curious 

 effects have been described, of which none are more remarkable 

 than those due to " half-vision," " half-blindness," or what is 

 technically called " hemiopsia " or "hemiopia." Many distin- 

 guished men have contributed information on this diseased 

 condition of the eye ; and it would appear from a paper read 

 before the Eoyal Society by Dr. Hubert Airy, that Wollaston, 

 Arago, Brewster, and Tyrrell describe one form of the complaint, 

 while Herschel, Wheatstone, Professor Airy, Dr. Hubert Airy, 

 and Dufour give the curious and minute details of another and 



