ZOOLOGY AND BOTANY, MICROSCOPY, ETC. 883 



refraction or reflection, in sucli a way tliat tliey emerge into air under 

 equal and opposite inclinations w', smaller than the semi-angle of 

 aperture of the dry objective above, which is focused to the point o' 

 of vertical intersection of the two beams. Every device which con- 

 forms to these conditions will act as an immersion iront-lens in regard 

 to the particular pair of beams in question. 



The conical or prismatic front will, moreover, like the hemisphere, 

 increase the power of the optical combination. This power may be 



7i Rl n w 



determined by the formula N = —. r where n is the index of the 



sm u 



glass front, u the internal angle of obliquity, and u' the angle of 



obliquity after emergence into air. In the ratio of the quotient N to 



unity the power of the objective will be increased (or its focal 



length diminished) in regard to the delineation of the particular set 



of lines from which the two opposite pencils originate. For any 



other set, of different closeness, u and u' will require different values, 



aad the power of the same cone or prism will be different. 



The ' Times ' on the Microscope. — The following leading article 

 appeared in the ' Times' of the 26th August: — 



" We publish this morning an article descriptive of some of the 

 progress which has been made during late years in the construction 

 and cheapening of Microscopes and of their accessory apparatus— a 

 progress so marked that it has become time for all who are engaged 

 in the work of instruction to consider carefully to what extent the 

 improved instruments of the present day can be employed for the 

 furtherance of the general work of teaching. If we may adopt 

 Paley's definition of education, as ' comprising every preparation 

 that is made in our youth for the sequel of our lives,' we shall be 

 prone to admit that few of these preparations can be of greater 

 importance, or of greater ultimate utility, than the training of the 

 eye to observe natural phenomena, and the training of the mind to 

 appreciate the meaning of these phenomena and their relations to 

 one another. It was a great day during the childhood of many who 

 have now passed the meridian of life when the lecturer with an oxy- 

 hydrogen Microscope was announced as being about to exhibit and 

 to discourse at the town hall ; and the huge ti-ansparency in which 

 the insect life of a drop of water was displayed in full activity 

 became a wollspring of new thoughts and of increased mental activity 

 t() nearly all of those who gazed in wonder at the presentment of 

 ra])id movement, of abounding life, and of continual destruction. 

 Tlie sight which was then to be seen only on rare occasions, and as 

 a sort of entertainment, is now at the daily command of every school- 

 master, or of every parent who can spare only a small amount of 

 money, and who possesses sufficient intelligence and manual dexterity 

 to learn the use of the instrument which, more than any other, has 

 led to increased knowledge of the structure of man and animals, and 

 to mcKlem improvements in the healing art. The powers now at the 

 disposal of the savant far surpass any which were attainable only a 

 ff;w years ago ; but the use of these high powers requires the devotion 



