448 H. von Mohl on the Arrangement of the 



natcd glass plate coincides with the object. When low powers 

 are used, this illumination may be too strong, which evil may 

 easily be remedied by altering the position of the condenser; 

 but when the higher powers are used, and the objects are very 

 delicate, the results obtained with it are surprising. It is evi- 

 dent that there is no occasion to darken the room in w^hich the 

 observations are made. 



But, as all have not the privilege of occupying a detached 

 house, and the opportunity of making use of sun-light through- 

 out the day, and as we are often without bright sun-light, it 

 would be desirable to find a substitute in strong artificial illumi- 

 nation. Whether this is possible, I do not know. Experiments 

 made with lamp-light did not yield satisfactory results. How 

 far the application of Drummond^s light might answer the pur- 

 pose, I have not experimentally determined. I should, however, 

 have no doubt that good results might be obtained with Drum- 

 mond's light, since those obtained with lamp-light are by no 

 means to be entirely despised; and the question is not one 

 of the application of light of the highest degree of intensity, as 

 with the solar microscope, — in the case of which, former experi- 

 ments convinced me that the light produced by the oxy-hydrogen 

 blowpipe and heated lime by no means forms a substitute for 

 sun-light. 



I may take this opportunity of making a few remarks upon 

 the use of plates of selenite in observations with the polarizing 

 microscope. When one of these is placed between the polarizer 

 and the object, as is well known (the polarizer and analyzer 

 being crossed), the dark field of the microscope becomes light, 

 and presents one of the colours of Newton's rings, according to 

 the thickness of the plate used. On viewing, under these circum- 

 stances, a doubly refracting object lying in the focus of the 

 microscope, this, in a certain relative position to the prisms, 

 appears of a different colour from that of the field, often of great 

 brilliancy, and which passes into the complementary colour, when 

 either the selenite plate or the object is rotated 90°. It is usually 

 stated that the plate of selenite is of important value when the 

 object but slightly polarizes light and therefore cannot be di- 

 stinguished in the dark field of the microscope, or exhibits merely 

 very obscure indications of radiation of light. In this case, after 

 the insertion of the selenite plate in such manner that the object 

 appears of a different colour from that of the field, and, on the 

 rotation of the plate, passes into the complementary colour, the 

 doubly refracting power of the object is more distinctly evident. 

 Now I will not deny that, in many doubtful instances, this pro- 

 ceeding leads to a decisive result ; but it is very frequently unsa- 

 tisfactory, because the selenite plates are too thick, and thus act 



