60 



Messrs. H. Hartridge and A. V. Hill. 



from the light source and forming a magnified inverted aplanatic image on 

 the slit of the collimator. The function of the second component was to act 

 as field lens ; it formed a magnified inverted image of a plane situated 

 between the two lenses of the first component on to the plane of the 

 collimator lens. The aperture of the condenser system was purposely made 

 considerably greater than was actually required to fill completely the aperture 

 of the collimator. The object of this system may be indicated as follows. If 

 a piece of plane parallel glass or a thin trough with plane sides containing 

 fluid be interposed between a lens and the plane at which it is forming an 

 image, but little disturbance will occur. If, however, the sides of the glass 

 be not parallel or if there is any lens action, then considerable alteration will 

 occur not only in the position of the image but also in its definition. As 

 will be explained in dealing with the measurements on the lens of the eye, 

 no matter how carefully the refracting power of the lens is neutralised, there- 

 will always be some residual refraction, particularly when waves differing 

 from one another greatly in length are to be measured. Now one property of 

 the condenser system employed is that it is to a considerable extent unaffected 

 by small changes in focus brought about by weak positive or negative lenses 

 placed between its components. 



The object of our earlier experiments was to ascertain the best way of dealing 

 with the different eye media. We found that the aqueous and vitreous 

 humour when placed in a small trough with parallel sides gave a clear sharp 

 image when a distant light source was looked at through the trough. With 

 "the lens and cornea this, of course, would not be the case. We tried two 

 ways of dealing with the former. The first was to take several lenses, dry 

 them superficially, and then squeeze them into the small trough, removing air 

 bubbles with a small glass rod. This method was quite unsuccessful ; the 

 difference in refractive index of the different zones of the eyes was found to 

 give a series of confused images of a distant light source. The second way 

 was to immerse the uninjured lens in some fluid of suitable refractive index 

 that would neutralise the convergence exerted by the lens on a parallel beam 

 of light passing through it. 



There are several groups of substances that could be used for this purpose ; 

 we had, however, to select one which, besides having the right refractive 

 index, also showed no marked selective absorption in the infra-red region. 

 After examining a number of oils and hydrocarbons of the paraffin series we 

 found in carbon tetrachloride the body most suitable for our purpose. We 

 found it to have no absorption bands over the range required, a result which 

 •confirmed Abney's* data for the same substance. Further, its refractive 

 * Abney and Festing, ' Eoy. Soc. Proc.,' vol. 38, p 77 (1884-5). 



