64 THE MICROSCOPE. 



as a result inherent in ou.r mode of viewing objects by 

 transmitted light ; nevertheless it is often productive of 

 serious embarrassment when we are dealing with the 

 almost inappreciable markings on the siliceous diatoms, &c. 

 The following general rule is given by Mr. Wenham 

 (Quart. Joicrn. Micros. Scien. 1854, p. 138) for securing 

 the most efficient performance of an object-glass with any 

 ordinary object : — " Select any dark speck or opaque por- 

 tion of the object, and bring the outline into perfect 

 focus ; then lay the finger on the milled head of the fine 

 adjustment, and move it briskly backwards and forwards 

 in both directions from the first position. Observe the 

 expansion of the dark outline of the object, both when 

 within and when without the focus. If the greater ex- 

 pansion, or coma, be when the object is without the focus, 

 or farther from the objective, the lenses must be placed 

 further asunder, or towards the mark ' uncovered.' If 

 the greater coma be when the object is within the focus, 

 or nearer to the objective, the lenses must be brought 

 closer together, or towards the mark ' covered.' When 

 the object-glass is in proper adjustment, the expansion of 

 the outline is exactly the same both within and without 

 the focus. A difierent indication, however, is afforded by 

 such test-objects as present (like the Podura-scale, the 

 Diatoms, &c.) a set of distinct dots or other markings. 

 For if the dots have a tendency to run into lines when the 

 object is without the focus, the glasses must be brought 

 closer together ; on the contrary, if the lines appear when 

 the object is within the focal point, the glasses must be 

 further separated." Mr. Wenham remarks upon the dif- 

 ficulty of accurately making this adjustment even in the 

 best made objectives. 



Mr. R. Beck, in a paper contributed to the proceedings 

 of the Royal Microscopical Society, directs particular 

 attention to avoidance of errors of interpretation. Viewing 

 objects by transmitted light, he observes, is a method 

 peculiar to the microscopist ; it is, however, one to which 

 our vision, applied in the ordinary way, is so much un- 

 accustomed, that in many instances it not only conveys 

 imperfect notions of an object, but produces appearances 

 which bear little or no resemblance to the true structure 



