fiSO VERHOEFF AND BELL. 



the lesion produced was well marked. It is therefore necessary to 

 illuminate a large area of the retina. This we have done by means of a 

 suitable system of quartz lenses used in connection with the magne- 

 tite arc as described on page 648. Intense illumination of such a large 

 area, however, for a long period of time entails a danger of over- 

 heating the fundus of the eye. This we have successfully obviated 

 by interposing a quartz, cell 5 cm. thick filled with distilled water to 

 absorb most of the infra red rays. If this had not sufficed, heat effects 

 could probably still have been prevented by interrupting the expo- 

 sures at intervals to allow for cooling to take place, a procedure that 

 no doubt would be necessary for light intensities only slightly greater 

 than that used by us. In fact in one of these experiments in which 

 the water cell leaked, a heat effect on the pigment epithelium was 

 actually noted (Exp. 88). 



As will be seen the system of quartz lenses employed concentrated 

 the light more intensely upon the cornea and lens than upon the retina. 

 Advantage was therefore taken of this fact to determine at the same 

 time the effect of exposures through various screens upon these struc- 

 tures, the results of which have already been given. None of the 

 screens obstructed any waves longer than 305 /x/x to 315 ^lyit, that is, 

 any waves that otherwise could have reached the retina through the 

 lens. The screens also pre^■ented excessive keratitis, which we de- 

 sired to avoid since it would have prevented us from later making 

 satisfactory tests of the lid reflex and pupillary reaction to light. 

 If these reflexes had been abolished this fact alone would have fur- 

 nished sufficient proof of the deleterious action of the radiations on 

 the retina. As a matter of fact, except immediately after exposure 

 a lid reflex was always obtainable. 



The details of these experiments are given on pages 655-658. 

 (Experiments 65 to 90). It will be observed that the exposure was as 

 long as one and one-half hoiu's in each of four experiments and one 

 hour in each of six experiments. In all except one experiment the 

 retinae were prepared for microscopic examination in the manner 

 already described (page 661). In Experiment 78 the eye was immedi- 

 ately opened and the retina bisected vertically through the optic disc, 

 one half being fixed in a saturated solution of mercuric chloride and 

 embedded in paraffin. The sections, 2 /x in thickness, were stained 

 in thionin as in the case of the other experiments. The other half 

 of the retina was used for vital methylene blue staining, the results of 

 which will be mentioned later in commenting on Birch-Hirschfeld's 

 observations (page 687). In none of the experiments could any 



