The basic characteristic that defines it is that it has two eyepieces. With this configuration the sample can be observed with both eyes simultaneously. Compared to monocular microscopes, this type of microscope generally reduces eye strain since it is not necessary to keep one eye closed to observe the sample. This feature makes them more suitable for working for long periods of time.
The configuration of a binocular microscope is internally very similar to that of a monocular microscope. The only difference is the presence of a prism that divides the light coming from the objective into two exactly equal beams towards the two eyepieces. Fig. 1. The objectives of binocular microscopes usually magnify between 4x and 100x while the eyepiece usually has a magnification of 10x or 20x. Therefore, the total magnification of binocular microscopes is usually between 40x and 1500x.
Binocular microscopes allow the distance between the two eyepieces to be adjusted to adapt it to the interpupillary distance of the observer. Fig. 2. In good quality binocular microscopes, the eyepieces can be individually adjusted to correct the observer’s eye defects and correctly focus the image of both eyes.
History of the binocular microscope
The first binocular microscope was built by the Capuchin friar Chérubind’Orléans in the 17th century. He himself had invented the binocular telescope shortly before and tried to extrapolate the idea from it to microscopes. This first binocular microscope was a forerunner of the stereomicroscope since it was based on using two objectives, each with a different eyepiece. The result was not totally satisfactory since the images observed with the Chérubind’Orléans microscope were not of good quality. This concept was more similar to the current binocular microscope because it was based on a prism system that allowed the image from a single objective to be divided. The result was also a three-dimensional image since the observed image was not exactly the same in the two eyepieces. However, for practical purposes the division of the light rays coming from the objective resulted in a reduction of its magnification capacity. For this reason, the scientific community continued to use the monocular microscope.
Binocular apparatuses from the end of the 19th century do not have the two symmetrical optical tubes, but the second one protrudes laterally from the vertical main tube. Fig – The rays from the target are separated by a prism, the Wedham prism, which is located immediately below the separation of the optical tubes. Fig. 3. This binocular vision is only achieved with low magnification, short focal length lenses in the English system ½ inch or more. The vision that was achieved lost sharpness with high magnifications, so with these binoculars the prism was extracted and then a monocular vision was had. Nos. 91 to 94.
Prisms in binocular microscopes
However, the idea of introducing prisms to modify the path of light inside the microscope had been a big step in the right direction. The definitive step came from the hands of the Leitz company thanks to the investigations of the physicist Felix Jentzsch and Ernst Leitz II. He was convinced that the creation of the binocular microscope was a necessary step for the development of microscopy. The design they developed incorporated a prism covered with a small mirror. This allowed dividing the beam of light from the sample in two without affecting the image quality obtained by the objective, so that the two eyes observed exactly the same image. In 1913 the new binocular microscope began to be commercialized, even today, binocular microscopes are based on the original idea of Ernst Leitz II and Felix Jentzsch. Fig. 4.
Difference between binocular microscope and stereo microscope
It is important not to confuse the binocular microscope with the stereomicroscope. While it is true that all stereomicroscopes are binoculars, not necessarily all binocular microscopes are stereoscopic.
The image observed in a binocular microscope is the same in both eyepieces, so that a two-dimensional image is observed. This is not the case with stereomicroscopes. This type of microscope uses two objectives and two eyepieces so that the image seen in each eyepiece is different. When looking at the sample, the two eyes combine the two images so that a three-dimensional image display is produced.
It is therefore evident that a stereoscopic microscope must always have two eyepieces, since otherwise it would not be possible to obtain 3D vision, this makes it a specific type of binocular microscope. Therefore, when we speak of a binocular microscope, we always refer to the compound microscope that has two eyepieces and a single objective.
Within the collection there are ten binocular microscopes: Nos. 17a, 90, 91, 91a, 92, 93 and 95 with the primitive system and 24b, 24c and 94 with the system developed by Leitz. These are set out below.