PKEFACE. y 



has been trained to the pursuit. And he has found himself obliged to 

 content himself with a mere indication of the new and important facts 

 now being brought to our knowledge by Microscopic inquiry, in regard 

 to the Deposits at present in progress on the bottom of the Deep Sea, 

 the Mineral constitution of Sedimentary and Igneous Kocks, and other 

 branches of Micro-Petrological inquiry, which are throwing a flood of 

 new light on the past history of the Crust of the Earth. 



It has been the Author's object throughout, to guide the possessor of 

 of a Microscope to the intelligent study of any department of Biology, 

 which his individual tastes may lead him to follow-out, and his indi- 

 vidual circumstances may give him facilities for pursuing. And he has 

 particularly aimed to show, under each head, how small is the amount 

 of trustworthy knowledge already acquired, compared with that which 

 remains to be attained by the zealous and persevering student. Being 

 satisfied that there is a large quantity of valuable Microscope-power 

 at present running to waste in this country, applied in such desultory 

 observations as are of no service whatever to Science, and of very little 

 to the mind of the observer, he will consider himself well rewarded for 

 the pains he has bestowed on the production and revision of this Manual, 

 if it should be tend to direct this power to more systematic labors, in 

 those fertile fields which only await the diligent cultivator to bear abun- 

 dant fruit. 



In all that concerns the working of the Microscope and its appurten- 

 ances, the Author has mainly drawn upon his own experience, which 

 dates-back almost to the time when Achromatic Object-glasses were first 

 constructed in this country. In his last Edition, he felt himself obliged 

 by the demands which were made by Official duties upon his time and 

 attention, to seek the aid of his friend Mr. H. J. Slack, in the prepara- 

 tion of the portion of the work specially relating to the Microscope and 

 its appliances. But having now, at last, the command of his own time, 

 he has preferred that this, like the rest of the Treatise, should be the ex- 

 pression of his own matured views; and has accordingly taken much 

 trouble to acquaint himself thoroughly with such recent advances, alike 

 in the theory and in the practice of Microscopy, as could be most fittingly 

 introduced into it. 



Accordingly, he has introduced at pp. 156-161 a concise account of 

 the f diffraction-theory ' of Prof. Abbe, which has now given the com- 

 plete rationale of the relation between the ( angular aperture ' of Object- 

 ives and their * resolving power.' And he has followed this up by a dis- 

 cussion of the question (pp. 161-173) whether the opening-out of the 

 angular aperture to its extremest limits is the end to be specially aimed- 

 at in the construction of Objectives for the highest kinds of Biological 

 research; in other words, whether an Objective which resolves the most 

 difficult Diatom tests, is on that account the one- best suited for follow- 

 ing the life-history of Monad, or for studying the development of a prob- 



