ILLUMINATION. 



XXV 



be not at hand. It is of little consequence which be selected, provided a large and firm 

 sloping arm-rest be furnished on each side of the stage. Either doublets or the lower 

 powers may be used. We have a very convenient simple microscope made by Mr. Varley. 



Leather Case and Collecting Bottles. The Diatomaceae, Desmidiaceae, and other smaller 

 Algae, as also the Infusoria, require to be collected and brought home in bottles. These should 

 be of about 1 or 2 ounces capacity ; and for portability without risk of being broken, they 

 should be packed in a case made of stout leather, with a separate space for each bottle. 

 The whole will pack up in the form of a book. These are manufactured by Mr. Ferguson 

 of Giltspur Street. 



Having given a sketch of the most important pieces of apparatus, we will say a few words 

 upon the illumination. 



Illumination. The best light in general for microscopic purposes is undoubtedly day- 

 light, or that of the sun reflected from the clouds ; and this is certainly the light which can 

 be borne for the greatest length of time without injury to the sight. The position of the 

 observer is of importance ; it should be such that the window is on his left hand, or even 

 the back slightly turned towards the window'. The advantages of this position are great, 

 for then but little light will enter the eyes directly from the window, and it is of the greatest 

 importance, during a microscopic examination, that the least possible amount of light should 

 be admitted to the eye, from any source, besides that transmitted through or reflected 

 from the object. In drawing also with the Camera lucida this position should be strictly 

 observed, for all extraneous light which would interfere with the distinctness of the 

 ipaage is thus excluded, and the shadow of the pencil and hand does not interfere with or 

 obscure the sketch in progress, which would be the case if the observer's right hand were 

 towards the window. But in daylight, the light entering the eye from the window, even 

 in the position above mentioned, will interfere with the observation, unless a preventive be 

 employed, which is to place a screen, either supported upon a stand or fixed to the upper part 

 of the body of the microscope, between the eye and the eyepiece of the microscope and the 

 light. This screen may be made of card-board or thin wood, covered with black velvet. If 

 it be fixed to a moveable arm, like the lens of the side-condenser, it may be easily placed in 

 any convenient position. If to be fitted on the microscope, it may be constructed thus : 

 a piece of stout card-board, of about the size and shape of one of the plates of this work, 

 should have the corners rounded off, and should be bent at a right angle at about the lower 

 one-fourth ; a hole being cut in the middle of the smaller portion, of a size just to fit the top 

 of the body of the microscope, a short tube of card-board is then made by sewing or pasting, 

 and this being fastened in the same way to the circular aperture serves to keep the screen in 

 position. The whole is then covered with black velvet. When used, the long flap should 

 be placed towards the left side ; it then shelters the eye and upper part of the eyepiece 

 from the light. A screen of this kind should always be kept upon the microscope, for it 

 is of the greatest service. A tube made of a roll of card-board, fastened to the inside of 

 the angle of the screen described above, will serve to fix it to the stem of the side-con- 

 denser ; it may then be made to slide upon this axis or stem at pleasure. It is hardly 

 possible to use the high powers of the microscope by daylight without a screen of this kind. 



But few persons have the opportunity of using daylight for microscopic researches, 

 hence artificial light of some kind is called into requisition, and the most common source 

 of this is an Argand lamp with oil. For ordinary purposes nothing can be better 

 than this. In a well-trimmed Argand lamp, the light is strong and perfectly steady two 

 essential requisites ; moreover, there is no great difficulty in obtaining sufficient light with 

 it when a power of three thousand diameters is used. A most intense light may be obtained 

 from the elegant little Camphine lamp of Messrs. Smith and Beck, and this is very advan- 



