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January 30, 1891.] THE PHOTOGRAPHIC NEWS. 91 axis the object would be kept in the field of view. Sir Howard Grubb makes a small telescope on this plan, and some years ago proposed a somewhat similar plan. A sketch of this plan I will show you. You will see, how ever, that here again the range is restricted, and to use the telescope, means would be required to constantly vary the inclination of the small mirror at one-half the rate of inclination of the short tube carrying the object glass. By the use of two plane mirrors, however, the solu- tionof the problem of a rotatingtelescope tube placed as a polar axis is solved. By having such a telescope with a plane mirror at an angle of 45° to the axis of the tele scope in front of the object-glass, we can, by simply rotating the telescope, see every object lying on the equator; and by adding another similar plane mirror at an angle of 45° to the axis of the telescope, as bent out at riyht angles by the first plane mirror, and giving the mirror a rotation perpendicular to this axis, we obtain the same power of pointing the telescope as we have in the equatorial. The idea of doing this was published many years ago, but it was left to the skill and perseverance of M. Loewy, of the Paris Observatory, to put it into practical use. He devised, and had made, a telescope on this principle, of 10} inches aperture, which was com pleted in 1882. It has proved itself an unqualified success, and many other larger ones are now being made in Paris, including one of twenty-three inches aperture, now nearly completed, for the Paris Observatory. A lantern copy of a drawing of this latter telescope will be thrown on the screen, in order that you may see what manifest advantages exist in this form of telescope. There is but one objection that can be urged—that is, the possible damage to the definition by the plane mirrors ; but this seems, from what I have seen of the wonderful perfection of the plane mirrors made by the Brothers Henry, to be an unreasonable one—at any rate, not an insurmountable one. In every other respect, except perhaps, a slight loss of light, this form of telescope is so manifestly superior to the ordinary form, that it must supersede it in time, not only for general work, but for such work as photography and spectroscopy. (To be continued.) • The Manchester Amateur Photooraphic Society.—A photographic exhibition in connection with this Society will be held at the Manchester Athenum (George Street entrance), on Monday, Tuesday, and Wednesday, February 9 th, 10 th, and 11th, 1891. Open Monday, 6 to 10 p.m. ; Tuesday and Wednesday, 10 a.m. to 10 p.m. Popular Afternoon Lectures.—The Council of the Society of Arts have arranged for a course of five popular lectures to be given by Captain W. de W. Abney, on the following after noons, at half-past four o’clock—February 13th, 20th, 27th, March 6th, 13th. The subject will be “ The Science of Colour.” The lectures will be of a popular and elementary character, and will be fully illustrated by experiments. The regulations for admission will be the same as for the Cantor Lectures, each member having the privilege of admitting one friend. National Association of Professional Photographers.— The first annual general meeting of this Association will be held, as announced elsewhere, at the Colonnade Hotel, Bir mingham, on the 12th of February, for the enrolment of members, election of officers, and the discussion of rules drawn up by the Provisional Committee. In order to facilitate arrangements for adequate accommodation, those who intend to be present are requested to send an intimation on or before February 11th to Mr. H. J. Whitlock, New Street, Bir mingham. patent IEntellgence. Applications for Letters Patent. 911. J. Prestwick and W. H. PRESTWICH, Warmington House, Tottenham, “ Gas or Oil Stove.”—January 19th. 998. C. Hoddle, 161, Camberwell New Road, London, “ Metallic Printing Frame.”—January 20th. 1,055. T. It. Dallmeyer, 115, Cannon Street, London, « Optical Projections of Photographs in Natural Colours.”—January 21st. 1,080. R. Schreiner, 8, Quality Court, Loudon, “Changing Boxes.”—January 21st. 1,101. G. F. Forster, 24, Compton Terrace, Islington, London, “ Double Front Combination for the Magic Lantern.”— January 21st. 1,130. J. H. Nestler, 23, Chancery Lane, London, "Hand- Camera.”—January 21st. 1,140. J. Armistead and F, T. Clark, “Lantern for Dark Room.”—January 21st. 1,187. C. Beck, 23, Southampton Buildings,London, “Cameras.” January 21st. Specifications Published. 669. January Uth, 1890.—“ Ascertaining the Requisite Time of Exposure in Photography.” Edward George Ballard, 1, Eversley Park, Chester, H.M. Inspector of Alkali Works. A ready method of measuring the amount of actinic rays proceeding from any landscape or object has for a long time been a thing much desired by photographers, both amateur and professional. Many instruments with this view have been from time to time brought out, but none of them have proved very satisfactory. The same may be said of tables, which, although of great assistance in open landscape, have this drawback, that, wherever the quality of the light is interfered with, that is, in forest scenery, deep glens with overhanging trees where the very air appears to be green, interiors of churches, etcetera , where the light which enters perhaps passes through coloured glass, and so forth, you are compelled to guess at the exposure after all. The apparatus about to be described purposes not only to measure the intensity of the rays of light proceeding from any object, but also the quality of the same. The principle upon which the instrument is constructed is based upon (1) that luminous calcium sulphide or Balmain’s luminious paint is only excited by those rays of light which usually act upon the photographic plate ; (2) that when lumi nous paint is excited by exposure to light, either daylight or other white light, such as a burning piece of magnesium wire, and the source of light is suddenly removed, it will be observed that the bluish light emitted from the luminous paint gradually fades. The rate and manner in which this fading occurs are employed by me in this invention as a means for ascertaining the intensity of the light proceeding from any object, landscape, or sea view. This is arrived at by noting the time required for the luminous paint to acquire the same degree of intensity as a portion of the light proceeding from the object, landscape, or sea view in question, and referring to a table specially con structed for that purpose. In carrying out my invention into practice, one form of appa ratus which I have found very successful is as follows, and is given by way of illustration only :— A wooden or other opaque tube, coloured dull black inside, and open at both ends, and of convenient size and shape at one end to adjust to the eye, is provided at the other end with a second and similar but shorter tube hinged to it, so as to form a lid. This lid is glazed next the tube with blue glass capable of cutting off all rays except those corresponding with those which proceed from the luminous paint. A second glass is slipped into this, coated as regards about half its area with an opaque layer of calcium sulphide paint (Balmain’s luminous paint) ; above this a number of layers of tissue paper or other uniform translucent material is placed, and a covering is added of ordi nary or cobalt blue glass. The number of sheets of tissue
