clxx Journal of the Asiatic Society of Bengal. [N.S., XIII, 



Interference Fringes formed by a grating. — By C. K. 



Venkata Row. 



A system of interference bands was observed extending throughout the 

 spectrum when using a celluloid copy of a Rowland grating mounted on 

 glass as a reflexion grating, the light being incident on the grating face 

 first. The theory of these bands is worked out and experiments given in 

 confirmation thereof. 



On the Interference Pattern of thin films at nearly critical inci- 

 dence. — By E. P. Metcalfe and B. Vexkatesachar. 



A somewhat convergent beam of monochromatic light is made to fall 

 on to a plane parallel air film enclosed between two glass plates immersed 

 in water at and near to the critical angle of incidence. The interference 

 pattern so formed is viewed in a telescope focussed for parallel light and 

 is found to consist of nearly straight parallel fringes of varying thinness 

 This is shown to be due to the rapid variation in the reflecting power of 

 the air-glass surface in the neighbourhood of the critical incidence. The 

 fringes have been photographed with light polarised in the two principal 

 directions. The theoretical intensity curves have been drawn and have 

 been found to agree with the photographs. A new set of interference 

 fringes is observed when the reflecting system La placed between crossed 

 Nicol prisms. These fringes have also been photographed and are 

 accounted for theoretically. 



A Note on the Efficiency of the Aeroplane Propellers : Meaning, 

 Measures and Tests, with an Outline of a new Method of 

 Procedure.— By S. S. Nehru. 



The following is a section-wise abstract of the above paper which i 

 a corollary to a technical paper , that has been submitted for publication 

 in Europe. 



Section 1. Introductory and explanatory. The disturbed motion in 

 the medium set up by the propeller is only a specific case of the turbu- 

 lent motion of fluids, which is the province of the physicist, and not of 

 the technician. In considering and measuring turbulence, the root- 

 problem of aerodynamics, an abrupt departure is made from the classi- 

 cal methods of the air-laboratorv, and the study of the efficiency of the 

 propeller is placed on a scientific basis. 



Section 2. The propeller, function, redefinition, best types. The 

 propeller is re-defined as a transformer, converting rotative into propul- 

 siTO horse-power. Leading types are considered from the standpoint of 

 efficiency :— (a) the Ratmanoff, subject of tests in England, (b) the 

 UiauviSre, the commonest in use, (c) the Bigourdan, a " circular wing/ 

 the efficiency of which in the laboratory is four times that of (d) the 

 Uuthemburg, the best German Type. 



Section 3. Considerations of efficiency, anomalies. A series of 

 anomalies are presented. Thus, the above types are fundamentally un- 

 like, and they are all supposed to possess the same high order of effi- 

 ciency ; classical methods, as shown by the technical reports of the 

 British Advisor Committee for Aeronautics, suppress turbulence, which 

 is precisely the mam characteristic of fluid motion ; anomalies in the 

 nrnn 1 Jr°^ m ^ re P orts are explained ; so also the anomaly of a 

 P ^MW % ' converting 50 HP into onlv HP 20 : etc. 

 of JnSw £^ tlc l sm of ******* conception of efficiency, and method 

 St £ w ?°wn ^w the current conception of efficiency * 



tKuS.1 L +£ a i bOV ; e \ and other anomalies I and the imperfections of 

 the classical method whuh measure the mere mechanical pull, in con- 



