272 



SCIENCE. 



[Vol. XXII. No. 563 



its exteriorly pure, white, trumpet-shaped, velvety flower 

 tinted with various clear colors of purple, golden, joink, 

 etc. Orchids iu great variety are numerous, also ferns of 

 all sizes, up to trees twenty feet high, are abundant. 



This wilderness contains much undeveloped wealth in 

 its export varieties of trees, medicinal and fibrous plants, 

 and in its undeveloped minerals, metals, and very fertile 

 agricultural lands, and has much to interest scientists, 

 especially naturalists. 



July 30. 



A NEW EEFLECTING AND DIRECT ACTING POLAR- 

 ISCOPE FOE THE ARC LIGHT PROJECTOR. 



BY OSCAE KKIPE, PHILADELPHIA. 



Refebring to a paj)er on the subject of Projection, pub- 

 lished lately in Engineering and several other periodicals, 

 it was then indicated that most of the accessory instru- 

 ments for Projection, among them the polariscope, would 

 become more po25ular and find increased employment in 

 the various courses of instruction. The arc light being 

 so convenient, prompt in application and so perfectly sat- 

 isfactory, suggests, of course, an extended application, 

 and in consequence the expert will frequently find chances 

 for imjsrovement. 



The favorite construction of the polariscope has been 

 with Nichol's Prisms, two of these being employed, one 

 for the polarizer and the other for the analyzer. To ob- 

 tain brilliant effects it is necessary that the former should 

 be at least two inches across the face; unfortunately it is 

 now impossible to obtain such large crystals of sjsar, and 

 as the demand for these instruments increases very much 

 the reflecting polariscope again comes to the front; the 

 old elbow arrangement furnished by some makers of in- 

 struments is a very clumsy attachment and inconvenient, 

 as it requires the projector to be turned side-ways so that 

 the light can reach the screen in front of the audience. 



Various modifications have been proposed mainly by 

 London makers and amateurs to obtain a direct acting re- 

 flecting polariscope by two opposite surfaces set in a box 

 at the usual angle and deflecting the beam upward or 

 downward, but the main objection, that of being incon- 

 venient, still remained. The optical bench of the Para- 

 gon Projector offers, however, special advantages in that 

 respect; the distance from the centre of the arc to the 

 slide base being sufficient to allow a downward j^olarizer 

 to be adopted, leaving abundant room for the object 

 stage, objective and analyzing prism upon the bench. In 

 practice this instrument is found to be simple in adjust- 

 ment with the light, and the results obtained are surpris- 

 ing; the field projected is perfectly circular and even, al- 

 ternating it light and completely dark by rotating the 

 analyzer. The object stage here used is a novel devise; it 

 consists of two uprights which open and close by a spring 

 forming a clamp, a rotating ring with spring clips is se- 

 cured to each clamp upright, so that three objects can be 

 combined at one time, which is required for circular and 

 eliptic polarization. The stage for exhibiting the phe- 

 nomena of i^olarization in crystal, glass forms (verre 

 trempe), and those produced by heating the object will 

 be described at a future time. 



The polariscope described above is sijecially adapted 

 for plane and circular polarization of geometric and 

 fancy designs of Selenite and Mica. The latter is eas- 

 ily obtainable and can be split into laminse of various 

 thicknesses, the thinnest that can be taken off in a 

 square of about two inches is technically known as an 

 eighth wave plate, the next thickness equal to two one- 

 eighth films superposed is termed a quarter wave film 

 and another equal to two one-quarter films superposed is 

 the half wave film. The quarter and half wave films are 



the most useful in producing the most marvellous color 

 combinations, imaginable, not only in the gay primaries 

 of the solar spectrum, but also in the more quiet grays 

 and plain colors generally; taking a specimen composed 

 of four or six strips of selenite about one-quarter of an 

 inch wide by one and a quarter inches long, laid closely 

 together, it will project its primary colors at once upon 

 the dark field obtained by the position of the analyzer; 

 the slightest turn to the right or to the left produces a 

 change in the colors, but if we move the jsrism through 

 one-quarter of a revolution the field is changed to a 

 ground flooded with light and the colors have respective- 

 ly changed to their complementary tint, the carmine has 

 become a pale green, the lemon color an azure blue and 

 so on; they are termed complementary because when su- 

 perposed they produce white light. Allowing the speci- 

 men to remain, we take advantage of the rotary slip in 

 front of our triple object stage and place there another 

 specimen of selenite strijjs exactly like the first, but place 

 it at right angles or diagonally and we now will have an 

 illustration of the fact alluded to that complementary 

 colors produce white light. The reason that only here 

 and there a square or diagonal of real black or white is 

 produced is found in the difficulty in matching exactly 

 the films. After passing through the various changes, 

 taking a note perhajos of the exact angle at which a cer- 

 tain color is produced so as to be able to repeat it after- 

 ward, we will remove the specimen from the front of the 

 stage, and rej^lace it by a quarter wave film; these have 

 generally the axis marked on the edge by an arrow. We 

 shall now obtain a decidedly different set of colors, which 

 con be varied by rotating the analyzer; but notice now 

 that instead of the two complementary colors we have a 

 continual interchange of four or more colors, which can 

 all be registered and rejieated. When the quarter wave 

 or half wave film is placed on the rotary clip at the back 

 and rotated we obtain a different set of colors as well as 



a colored background. A sjjecimen representing three or 

 four concentric circles, or a wheel divided into a number 

 of sections joining at the centre or again a thin slab of 

 selenite which is ground concave on its face, either of 

 these will give the most beautiful and fascinating changes 

 of color. As these various types of colors are absolute 

 standards taken from the book of nature which can be 

 exhibited precisely alike, it is obvious that we have here 

 in this branch of polariscope study the most brilliant, 

 complete and unchangeable system of color samj)les with 

 their complementaries and color contrasts which far sur- 

 pass any book of artificial colore. These when projected 

 on the screen in a class become the objective point of 

 every member, and can be pointed out, and commented upon 

 -by the instructor. As the geometric designs may be 

 varied in composition, the mica films being very inex- 

 pensive, it requires merely a little patience and experi- 

 ence to produce an unlimited variety. The apparatus 

 described in this article is made by Queen & Co. Incor- 

 porated of Philadelphia. 



