riment. I selected a which worked as an immersion, as this position prevents confusion concerning other points of adjustment. The full aperture, as measured by the 'sector through a slide with water between that and the front lens, was 120°. The focal distance as immersion was 041. The diameter of transmission on the surfaces of front lens was 07, ascertained by allowing a drop of milk to dry on the front and measuring the diameter of the light spot from parallel rays entering from the back, using a low-power object-glass and micrometer eye-piece for the measurement. The field of view included a diameter of 03 on a stage micrometer. With the exact focal distance from the front lens as a starting point, it remained to ascertain what were the apparent apertures taken through various stops of definite diameter set close to the front, that could only admit the base of a cone of rays from an angle proceeding from the axial focal point up to a known diameter of stop. The arrangement that I use is a form of adjustable slit, consisting of two strips of very thin platinum foil; one piece is cemented on to a slip of thin plate glass, which is made to slide under two staples by a micrometer screw acting against a counter spring. The fixed strip of foil is attached to one of the staples, so that when the screw is quite home the edges meet. The various widths at which the instrument was set were measured under the microscope with an eye-piece micrometer. Having got the desired width, the object-glass to be measured was attached, and the body of the microscope lowered till the slit came in contact with the front lens, a drop of water having been placed over the slit to prevent undue refraction, and obtain more light. The apparent angles included by these limiting edges or stops were measured by the usual sector method, of rotating the microscope on a turn-table graduated into degrees, and ascertaining the vanishing point of a distant light; or, preferably, by means of an examining lens at the eye-piece, for observing the disappearance from the field of an actual image. The real or true angles were estimated from the distance of the focal point, up to the known measure of the edges of the stop. Avoiding fractions of degrees, the following table gives the comparative results: The last item obtained with a stop of an inch in diameter, indicates an aperture of 50° by the usual methods, whereas the true angle is only 30, an error being shown in excess of more than fourteen times. The annexed diagram demonstrates the cause of The central angle shown is the true aperture assigned this error. 03 .002 by the small stop at the base. The oblique angles represent the pencils, including the field of view, and showing light or an image at the eye-piece, up to an angular range of 50°. It is these outside rays which are superadded to angle of aperture taken by the usual method, and which are the cause of erroneous indications greatly in excess of the true angle. The foregoing is a mere illustration of excessive angles, indicated from oblique pencils, or angle of view alone, irrespective of true angle of aperture, for, of course, limiting stops of known diameter placed on the front lens serve no purpose for measuring full central angles. The difficulty of estimating the degrees of these angles accurately, by such minute measurements as the focal distance, and working diameter of the front lens, may be avoided by halving this as an unknown quantity, and obtaining the value of the central angle by the differential results, shown between a half-obscured front surface and an entire one. That which is understood (and, in fact, always has been) as a definition of angle of aperture, in a micro object-glass, is a triangle having a base equal to the available diameter of the front lens, and a height equal to the focal length measured therefrom. Now it is clear that if half the front lens is stopped off diametrically close to the surface, only half the base of the triangle must remain, and consequently but half the existing angle or cone of rays in the axial direction. By the sector measurement such is not shown to be the case. The apparent degrees from a half diameter of incident front surface are much in excess of a half quantity, because the rays that form the oblique pencils extend behind and under the half stop, nearly to the margin of the field of view beneath. These are beyond the true axial angle of aperture, and are the cause of the false quantity always measured in excess of the proper angle, the rays including the angle of field must therefore be deducted up to the centre. This the half stop enables us to accomplish. Using the sector measurement the rule is this. Subtract the degrees shown by half the lens from the degrees of the entire lens, and twice the difference is the aperture. By this simple rule we eliminate the angle of field, which has hitherto been erroneously added to angle of aperture. In order to obscure the half of the front lens, take a small piece of tinfoil with a clean-cut edge, lay this on a glass slip, and smear its upper surface with a bit of dry soap (anything moist will bedew the glass), place this on the microscope stage, and bring the edge in focus under the object-glass, until it exactly bisects the field vertically. Then lower the object-glass on to the foil, which will adhere to the front lens. The microscope body may then be laid horizontally, and the apparent aperture of the open half of the lens measured by the sector, which invariably indicates considerably more than half of the full aperture, taken by the same method. In objectives of large angle, twice this half measurement will amount to absurd and impossible angles. The application of the above rule will require to be thoroughly investigated before it can be finally adopted. In every case it brings out a reduction on the degrees of aperture indicated by all the modes of measurement in present use. I give its test, on what I have repeatedly stated to be fabulous apertures, viz. 180°. I have one of these objectives-a-yet in ridiculous disproof of such an aperture (which would bring the focal plane on to the surface of the front lens), it is remarkable for its working distance, and will penetrate through the cover of any object in my cabinet, and used with thick covers, either as immersion or dry, its performance is very fine. Taken by the sector, the open aperture, or what I will term angle of field, is 180°. The aperture of half this lens is 115°, twice the difference is 1302; this represents the degrees of the central pencil, which is the true aperture. I take another example of an immersion lens, in which the maker actually claims "plus 180°." On trying this by the sector, the light image vanished at a range of 178°, beyond which it was totally obscured. The aperture of half the lens was 114°, twice the difference is 128°. IV.—Improvements in the Micro-spectroscope. (Read before the ROYAL MICROSCOPICAL SOCIETY, December 13, 1878.) PLATE XVII. EARLY in this year we were reminded by Mr. Crisp that one of the objects of this Society was the promotion and diffusion of improvements in the optical and mechanical construction of the microscope, and as during the last twelve months I have been making some experiments with a view to modifications of the micro-spectroscope, and have just received from the maker an instrument which differs in several particulars from the usual form, I have ventured to bring it before the Society, and after pointing out the features in it which I believe to be new, shall leave it in your hands to decide whether it is an improvement or not. One of the first things which struck me as being an imperfection in the ordinary instrument was the slit, and the facility with which its jaws collect dust, thus causing those objectionable black lines across the spectrum, and though when the spectroscope is to be used in conjunction with the microscope the slit is much more protected from dust than in a table spectroscope, yet, nevertheless, from its internal position it is more difficult to get at, and sometimes little particles remain after several attempts to remove them. I cannot claim to have done much work myself in this branch, but I am told that with an instrument in constant use, the slit is liable to get out of order, and requires adjustment to maintain the perfect parallelism of the jaws and the integrity of their opposed edges. As soon as I began to make inquiries on the matter, I found I was by no means the first who had recognized these difficulties, and that during the last twelve years others had been considering what could be employed as a substitute for the slit. Returning one night from one of our meetings, it suddenly occurred to me that if a rectangular glass prism was taken and the apex of the right angle carefully ground off and the resulting face polished, it would transmit a narrow pencil of light, which would work admirably in spectroscopic examinations, any light falling upon the sides of the DESCRIPTION OF PLATE. FIG. 1.-New micro-spectroscope. 2.-Plan of internal platform, showing prism fitting, with three prisms in situ, comparison prism with slot, &c. 3.-Tube carrier for comparison stage. * Journal of the Royal Microscopical Society,' vol. i. p. 121. |