310 



KNOWLEDGE. 



Auoust, 1913. 



edge of the storm to the centre. In the tropical storm there is 

 a moderate decrease of pressure to within forty or fifty miles 

 of the centre, and then a rapid fall, which in exceptional cases 

 may descend to twenty-eight inches and even less. This 

 steep-pressure gradient marks the area of the destructive 

 winds and the excessively heavy rainfall which are character- 

 istics of the tropical storm. 



CLIMATE AND HEALTH.— During last session Mr. 

 W. Marriott read a paper on " Meteorology and Public 

 Health " before the Institute of Sanitary Engineers, which 

 has now been published in their Journal. He showed that a 

 bracing climate is invigorating and acts as a tonic to the body. 

 This is due to an open exposure and a good wind or current of 

 air blowing over the place. This continuous flow of air over 

 the body removes the moisture from the skin, and so causes a 

 lowering of the temperature, which in turn produces a crisp 

 and pleasant feeling. A relaxing climate, on the other hand, 

 is enervating and causes a languid feeling. This is largely due 

 to the situation being sheltered, and also to comparatively 

 little movement of the air. Consequently the moisture from 

 the skin is not so freely removed as when there is a continuous 

 flow of air. Again, there is the inland climate and also the 

 sea-coast or maritime climate. Inland the temperature, as a 

 rule, is high during the day and low during the night, and so 

 there is a considerable diurnal range of temperature. Along 

 the sea-coast the temperature does not rise so high during the 

 day nor fall so low during the night ; consequently the range 

 of temperature is less on the coast than it is inland. 



Extremes of temperature — hot weather in summer and frost 

 in winter — cause an increase in the death-rate. In summer 

 the increase in the number of deaths is due almost entirely to 

 cases of infantile diarrhoea. In winter the increase in the 

 number of deaths is due almost entirely to diseases of the 

 respiratory organs, especially among young children and old 

 people. 



Pneumonia and bronchitis are most prevalent in the colder 

 months, and generally follow cold, damp weather with marked 

 changes of temperature, which lower the vitality and are con- 

 ducive to chills. Severe cold spells are likely to be followed 

 by an increase of pneumonia, especially among elderly persons 

 and children. 



MICROSCOPY. 



By F.R.M.S. 



THE APLANATIC AND ACHROMATIC CON- 

 DENSER AND ITS USE AS AN APERTOMETER.— 

 The Aplanatic and Achromatic Condenser possesses another 



useful quality, inas- 

 much as it can be 

 employed as an aper- 

 tometer. This pro- 

 perty arises from the 

 following optical facts. 

 The iris - diaphragm 

 being situated near the 

 lower focal plane of the 

 condenser, an image 

 of it is formed thereby 

 at a distance which 

 may be equated at 

 infinity. Of this image 

 the objective forms 

 another at its posterior 

 focus and situated near 

 the outer surface of the 

 back lens on the side 

 towards the eyepiece. The rays of a pencil of light entering 

 the condenser in a direction parallel to its axis intersect at its 

 anterior focus. As this is made to coincide approximately 

 with the lower principal focus of the objective it follows that 

 the pencil of rays which proceeds from the focus of the 

 objective will emerge from its posterior focal plane in the 

 form of a pencil composed of approximately parallel rays. 



Figure 339. 



The base of the pencil of rays appears on the back lens of the 



objective as a bright circle, and hence it will be seen that this 



bright circle of light and the image of the iris-diaphragm are 



situated more or less in the same plane. The diameter of the 



circle of light is governed by the aperture of the objective 



which corresponds to an equal angular aperture of the 



condenser. The latter again is determined by the opening of 



the iris-diaphragm. The aperture of the condenser can be 



computed from the diameter of the iris-diaphragm, thus 



supplying the requisite datum for determining the aperture of 



the objective. 



That the condenser may be used in this way as an aperto- 



meter is due to the fact that its formula satisfies the sine 



condition, and not so much to its spherical and chromatic 



correction, the principal effect of the latter being that the 



condenser furnishes a more sharply defined image than an 



ordinary condenser. 



i 



The sine condition is expressed by the formula = const. 



sine a 



In the case of parallel incident light this constant quantity is 

 numerically equal to the focal length of the condenser, h is 

 the semi-aperture of the iris-diaphragm, whilst sine a with dry 

 lenses and m sine a. with immersion lenses represents the 

 numerical aperture of the condenser. From this it will be 

 seen that the aperture of the condenser varies directly as the 

 numerical aperture of the condenser as well as that of the 

 objective. This simple relation is a convenient circumstance, 

 inasmuch as the indices corresponding to the numerical 

 apertures 0-1, 0-2 . . . to 1-3 are separated by uniform 

 intervals (see Figure 339^. 



To calculate and register the scale of apertures become 

 under these circumstances a very simple matter. When a 

 preparation lies between the objective and the condenser the 

 angle by which the rays emerge from the condenser is the 

 same as that by which they enter the objective ; that is to say, 

 the angles which determine the aperture of the condenser and 

 objective remain equal, since the object slide, the cover-glass, 

 and the interval between the latter and the objective behave 

 optically as plane and parallel plates. This being so, the 

 angles of incidence and emergence of a ray passing through 

 the intervening media are identical so long as the surround- 

 ing media — in this case air, water, or oil — remain the same. 



The aperture maybe determined in the following manner : — 

 Focus the objective in the usual manner in the plane of the 

 object, using diffused light ; next remove the eyepiece and, 

 placing the eye immediately above the tube, view the circle of 

 light at the back of the objective and open the iris-diaphragm, 

 the image of which will also be seen in this plane, until its 

 opening just coincides with the boundary of the circle of light 

 at the back of the objective. To use the condenser in this 

 way it is necessary to have the mount of the iris-diaphragm 

 graduated in terms of the numerical apertures. 



It will be admitted that it would be difficult to imagine a 

 simpler and more inexpensive device for measuring the aper- 

 ture of lenses, and the manner of using it cannot give the 

 slightest difficulty. The accuracy of the results is all that could 

 possibly be desired for any practical purpose. The difference 

 in the aperture of objectives, as found by this simple method, 

 and its absolute value, as determined by the most exacting 

 methods in use, is too insignificant to disclose any appreciable 

 difference in the light-transmitting and resolving power of 

 objectives differing to this small extent in their apertures. As 

 a matter of fact, these discrepancies are no greater than those 

 unavoidable and invariably existing differences in the in- 

 dividual objectives of the same denomination arising from 

 slight variations in the thickness, position, and mounts of the 

 lenses. 



It is no doubt convenient to be able to determine without 

 trouble and with practically a sufficient degree of accuracy 

 the aperture of an objective. This, however, is scarcely the 

 best use to which an apertometer can be put, as it is a far 

 more important advantage that the apparatus enables the 

 observer at any instant to ascertain and state numerically the 

 aperture of direct light at which he is actually working, and 

 that will give him the best results. 



