50 



Dr. Matthews had hit upon the same plan, so that he hoped that what he 

 had to say might be considered as a joint communication. The object to 

 which he referred was shown under polarized light, and he wished to sug- 

 gest that polarized light might be of use as an addition to staining for 

 vegetable and some animal substances, as it seemed to differentiate tissues 

 somewhat in the same way. In practice it might be found to have its dis- 

 advantages, but it might have its advantages. No special preparation of 

 the tissues was required, and the conditions were more natural than if they 

 had undergone the process of bleaching and staining. Then it would be 

 possible, when they had a known selenite, always to repeat the same effect 

 when required, whereas staining frequently faded, and if there were any 

 doubt as to the meaning of what was seen, the effects conld be altered, 

 and results secured that would be unattainable with the fixed effects of 

 double staining. There was, of course no difficulty in getting triple staining, 

 or producing various colours, but the object which he had shown was as if 

 stained with a single colour only. He wished also to say that the object 

 was shown with oblique polarized light on a black ground. He had heard 

 some discussion as to the best means of obtaining polarized light on a black 

 ground, and had heard it suggested that the results depended entirely on the 

 object, that it was to be obtained only now and then in the case of certain 

 objects which had a capacity for it, also that it depended on the size of the 

 polarizing prism and other causes. No doubt these things did affect it to 

 some extent, but he was of opinion that the effect was largely a question of 

 what the object was mounted in. He did not find that Canada balsam was 

 the best medium ; in fact, the best effects were obtained by mounting iu 

 glycerine, when there was very little difficulty in making out the details, 

 and the object looked brighter upon a blacker ground as contrasted with its 

 appearance when mounted in balsam. He thought the idea would be found 

 worth attention, especially where it was desirable to examine objects under 

 various conditions of direct and oblique light. 



The President said they must all feel very much obliged to Mr. Michael 

 for this communication, but so far as he was concerned, he had always 

 found a good deal of difficulty in using polarized light on objects mounted 

 in glycerine. 



Dr. Matthews said he had but little to add to what Mr. Michael had 

 said so well. On one point, however, as to the superiority of glycerine over 

 balsam for this kind of examination, his experience was rather the reverse 

 of Mr. Michael's. Whether this arose from any difference in the objects he 

 could not say, but he thought that the effect was probably due to some 

 difference in the density of the objects ; the only way of settling the point 

 would be to mount the same object in both ways. He should also say that 

 if they got extremely oblique light, they got also fringes of colour, probably 

 owing to diffraction. Mr. Michael had been very successful in getting dark 

 ground illumination, but there seemed to be some curious effect produced 

 by a spot lens, less colour being produced in that way than without, although 

 it might have been supposed that the contrary would be the case. As to 

 the differentiation of tissues, precisely the same effects were produced as 



