410 Dr. F. Semon. On the Position of the 



extraneous causes influence his respiration, the state of the glottis 

 during tranquil breathing remains pretty constant, as I have ascer- 

 tained by measuring several persons on different days, and by com- 

 paring the results thus obtained. 



As to the method, I availed myself of a mirror on which a millimetre 

 scale is engraved in such a direction that when the mirror is held in 

 the correct position the scale stands parallel with the plane to be 

 examined, that is to say, the distance between the inner borders of 

 the arytenoid cartilages. The source of confusion which this arrange- 

 ment undoubtedly entails, viz., that the marks of division of the scale 

 are themselves reflected in the mirror if the latter be held in the 

 correct position, that is to say, at an angle of 45 towards the horizon, 

 is easily eliminated by a little practice. 



There is. however, another point of the greatest importance with 

 regard to the exactitude of the measurements, and one which, with 

 the exception of a passing remark of Mandl's (loc. tit., p. 116), I find 

 to my surprise is not mentioned by any previous observers who speak 

 about the use of these graduated mirrors ; I refer to the considerable 

 difference of the actual from the apparent length of the distance 

 measured. The distance between the inner surfaces of the arytenoid 

 cartilages, as apparent on the scale engraved on the laryngeal mirror, 

 is not equal to the real distance, but considerably smaller than this, 

 and the proportion between the real and the apparent lengths is, 

 according to the principles of physiological optics, the same as the 

 proportion between the real distance of the object, on the one hand, 

 and the distance of the mirror, on the other, from the observer's eye 

 respectively. 



The real distance of the object (i.e., the level of the glottis) from 

 the observer's eye is of course equal to the distance of the observer's 

 eye to the centre of the mirror plus the distance of the centre of the 

 mirror to the object. 



If we therefore assume, with B. Frankel (loc. cit., p. 16), that the 

 average distance of the observer's eye from the centre of the mirror is 

 22 cm. (of which 14 cm. go to the distance to the mouth of the person 

 examined, and 8 cm. to the distance from the mouth to the mirror), and 

 that in an adult man of middle size the average distance from the 

 centre of the mirror to the glottis is 8 cm. (in an adult woman 6 cm.) 

 we obtain, if we call the apparent lengths of the base of the glottic 

 triangle as seen during quiet respiration in the mirror a, and the real 

 length which is wanted a;, the following proportions*: 



* The same result is obtained if the ratio of the size of the scale on the mirror 

 to the real size of the object be enquired into. If the distance from the eye to the 

 mirror = A, and the distance from the mirror to the object = B, the whole 

 distance from eye to object = A + B ; the apparent size of the object will 



be to that of the scale on the mirror as . 



A + ji 





