OF THE CHARACTERISTICS OF THE DIATOMACE.E. 49 



confusion among authors, who give the most contradictory 

 estimates of the numbers contained in 1*1000 inch. 



I think it is more than probable that, when the highest 

 powers are being used, secondary representations, or images, 

 of the striae appear before the eye, owing to a possible dis- 

 tortion of the rays of light, and that these images deceive 

 the observer, and induce him to note down a larger number 

 than really exists. 



The coarser lines may be measured by means of an 

 ordinary stage micrometer ; but, for the finer structures, an 

 eyepiece micrometer must also be employed. In either 

 case, the operator must not be content with a single obser- 

 vation of the striae, but must reckon them several times, so 

 as to be able to draw an average, and get as near the truth 

 as possible. 



As however the frustules, minute -though they are, have 

 a certain thickness sufficient to interfere with iheir tran- 

 sparency, a special expedient must be resorted to to obviate 

 this inconvenience ; otherwise the valvular markings can 

 never be seen so clearly as is needful in the operations just 

 spoken of. This may be managed by loosening the hold 

 which the hoop, or outside frame, has on the two opposite 

 valves, thereby setting them free, and enabling the observer 

 to view each one as a separate object. 



To effect this, let the frustules be boiled in nitric acid, 

 to which chlorate of potassium is to be added in small quan- 

 tities. After the boiling has continued for about a minute, 

 they must be well washed in soft water, in order to get rid 

 of the acid. If this be carefully done, the intercellular 

 substance which binds the valves together is dissolved, and 

 the latter fall apart. 



[For the various methods of mounting the Diatomaceae in 

 balsam and fluids the reader is referred to works specially 

 devoted to the microscope. There is the less need to intro- 

 duce the subject here, as it has been already exhausted in 

 Mr. T. Davies' admirable manual, c The Preparation and 

 Mounting of Microscopic Objects.' Hardwicke. — Ed.] 



